There are many different types of pickup trucks and it's important to determine which type is appropriate for your illustration. Here are a few different examples to show the range of body types. You wouldn't want to draw a massive, modern, 4X4 truck in a vintage setting, so think about what is needed for your illustration before you begin.
A lot of people make the mistake of assuming that a pickup truck is just a car with a truck bed in the back rather than back seats. This is hardly the case (except for some vintage models). Here is a side by side comparison of a car and pickup truck. Pickups are made for hauling loads in their bed, so they are considered to be work vehicles, and their appearance exemplifies this. Notice how the truck is "boxier" and larger. The engine compartment on trucks are especially larger and pickups are typically much higher off the ground than cars.
To draw a pickup truck, let's begin by laying down our basic guide lines. These lines do not have to be perfect, and they should be drawn very lightly with a 2H or harder pencil so that they are easily erasable later on. Draw a rectangular cube shape for the lower part of the body. Then draw another cube shape with a wedge taken out of the front for the upper part of the cab. These are out basic guide lines.
Drawing vehicles of any kind can be very difficult for most people. It is important to remember is that, when looking at a vehicle straight on, both sides of a vehicle are identical to one another. They mirror each other. Keeping that fact in mind, let's draw the front bumper first. It's also important to remember that some parts of a vehicle, such as the front end, are rounded slightly. This means that we have to draw the bumper with a curvature to it. Also, because of the curvature, the features on the far end of the bumper will have a different perspective and be smaller than the feature closest to us. Draw a couple of turn signal lenses on either end of the bumper, along with some decorative trim above them. You can add more character by adding a decorative grill to the lower portion of the bumper, and also add additional lights (or fog lights) mounted in the middle.
Next, draw the grill of the truck just behind the mounted lights. The grill should be fairly simple, with just a few bars stretching across the width of the grill, rather than a bunch of crisscrossed lines. On either side of the grill, draw the primary head lamps, which should be slightly square shapes. The parking light on the far side has wrapped around the body of the truck, so we cannot see it at this angle. We should, however, draw the parking light nearest us, which connects directly to the headlamp. Don't forget to draw an emblem in the center of the grill. You can add more character to the truck by adding a bug shield mounted to the front of the hood area.
Next, let's draw the basic contours of the truck's body. Draw the top of the hood first, which will end near the base of out wedged cube guide. When drawing the base of the vehicle, be sure to draw the wheel wells. It is important to know the proper location of the wheel wells. The front one begins directly behind the edge of the front bumper so that the weight of the engine rests directly on the axle of the front wheels. The rear wheel well rests just in front of the rear end of the vehicle. It is important that these features are draw in the proper location, or else the truck will look very awkward. Close off the bed of the truck in the back.
Then draw the basic contours of the upper part of the cab by using the wedge cube guide lines for reference. Notice how even though the basic shape of the vehicle is very "squarish," the body has many rounded and curved contours to it.
Let's move on to drawing the windows. You can add more detail by including weather-stripping to the side window. Also leave a small triangle of plastic for the side mirror to mount to in the next step.
Add more feature such as the windshield wipers side mirrors. Be sure to draw the passenger side mirror peaking up from the far edge of the truck. Draw the door handle and gas cap. These features make the image more believable and lifelike.
Add in some decorative grooves down the sides of the truck and around the wheel wells. Also draw the door of the truck while you're at it. Be sure that the door properly molds to the shape of the truck's body.
Drawing the tires can by rather tricky. To do it properly, draw squares in the location of the tires. The tops of the squares should start in the middle of the wheel wells and extend well beyond the base of the vehicle. Then draw an X from corner to corner inside each square. Then, but using the intersection point of the X, draw another X in each square from top to bottom and side to side. This will find the center of each side of the square and act as a guide to create the circles.
To draw the actual circle, draw the lines of the circle through the points where the inner X meets the sides and tops of the squares. This will create properly proportionate circles, but they are not yet three dimensional.
To make the circles three dimensional enough to look like tires, use the above methods to create a partner circle next to your original circle. Then, draw a line from the top edge of the first circle, to the top edge of the new circle, as seen here with a green line. Do the same for the bottom of the circles.
Then darken the proper lines and erase all unneeded lines. You should now have disk or cylindrical shaped objects which resemble the basic shape of tires. You can use the same method to draw the front, passenger side tire. However, this third tire will likely be blacked out by shadow later on, so if you're confident enough to eyeball it, go for it!
Add more detail to the tires be added tread. Make the tread as simple or as complex as you'd like. Larger groove in the tread suggests that the vehicle is used for off-roading. Very smooth treads suggests that the tires are worn and the vehicle has been driven a lot (so it’s important to not give smooth treads to a brand new vehicle).
Finish the wheels by drawing the rims. The lug bolts mount in the center of the tire and the arms of the rims reach out and hold the tire. Don't forget to add in mud-flaps while you're at it.
We are almost done, but we need to draw the front and rear axles peaking down from underside of the truck. These don’t have to be super detailed, since we will shade them in the next step. Without them, however, the truck looks unfinished and less solid.
Next, shade in the wheel wells, the axels, and the third tire. This gives the image a sense of depth and perspective. You can add additional shading if you wish, but be sure to establish a light source.
Finally, ink your image using either a brush and ink, or Micron markers. Use a ruler and, if available, a flex curve. Be sure not to ink any unneeded guide lines or sketch marks. After the ink has dried, erase your pencil marks. You should now have a nice, clean image of a pickup truck. Good work!