To start out you will need a model. You could make your own, or you could download on from a model sharing site. Since I don't have much modeling skill, I went the download route. Also note that Trigger Rally can only render triangles properly.
Trigger Rally only allows one texture per object, so if your model has more than one texture you will need to combine the textures.
Next you're gonna want to adjust the UVs so that they match your vehicle. Your gonna need your blender layout to look a little like this.
Next you're gonna want to deselect all in the 3D viewport. Then select the next material like before. Adjust it to match the texture. Do this for all the materials.
Next you will want to scale the truck to match the other vehicles in the game. What I do is import a car from the game and then scale my vehicle to match it. Blender likes to rotate .obj models when importing, so make sure to clear any rotation. You will also want to clear any rotation and movement off of the vehicle you're working on. Then enter edit mode an the vehicle, select everything and scale it so that it's to scale with the vehicle from Trigger.
Now that you've done this you can remove the reference vehicle or move it to another layer.
Now you will need to edit the wheel. It's easier to do if you move it to another layer. Enter edit mode. Select all and snap it to the 3D cursor. I'm assuming your 3D cursor is at 0,0,0. If it isn't, move it first.
Trigger Rally doesn't flip the wheels for the other side of the car, so your wheel will need to have the hubcap on both sides. Delete the back side of the wheel. Then select the front side of the wheel and copy it. With the pivot point at the 3D cursor, scale -1 on the x-axis. Then flip the normals on the piece you just copied. Then you will want to merge the edges of the two pieces. Then select the whole thing and snap it to the cursor again.
Now delete all but one material. Name the material something memorable like makeamericagreatagain. Then on the textures tab set the texture to your texture file. Make sure it's the only texture. Then do the same on the car body.
Congratulations! You are ready to export.
Now go to the folder you exported the files to. Open the .mtl files in a text editor and delete part of the file paths for the textures so that it's just the filename as shown below.
Now go to the Trigger Rally vehicle folder. Copy one of the .vehicle definitions files to the folder you exported to. Open it in a text editor.
Change wheelmodel to the name of your wheel model. Change the model for the body to the name of your model. You'll also want to change some of the other stats like name, class, locked, weightkg, enginebhp, wheeldrive, handling, mass, and powerscale. Note that weightkg, enginebhp, wheeldrive, and handling are just for display and don't actually change anything about the vehicle. To actually change weight you need to change the property mass. To change the power you need to change the property powerscale. As for the wheels I'll cover that in a second.
Save the file. Then copy your texture to the folder. Now you can copy the whole folder to your Trigger Rally vehicles folder. Load up the game and test it out.
As you can see the wheels aren't in the wheel wells. This can be adjusted by editing the property "pos" in the wheel tags in the .vehicle file.
I believe that the "clip" tags control the hitbox. I haven't tried editing them yet.
See my Jeep in action
Download My Vehicles