Like the randomiser, with none of the random! Based on my randomiser mod, this lets you easily play single car challenge runs.
Download the file from the releases page
Install into your Hit & Run mod launcher Mod folder Documents\My Games\Lucas' Simpsons Hit & Run Mod Launcher\Mods and open mod settings and pick your car. It'll spawn as the default level car and as every forced car.
You can also enable settings to skip the requirements to have a certain car or costume for certain missions, and to skip all FMVs except the intro cutscene (which can be skipped with the Skip Intro Cutscenes mod instead).
This mod will probably not have much more worked on it, but was a simple adaptation of the randomiser! Enjoy