Since I can finally play I76 again thanks to the awesome people at GOG I figured I'd share a simple trick to get almost any custom vehicle you want as the main vehicle in TRIP mode.
First you make the vehicle customized to your liking in the Scenario section(From the main menu go to MELEE-->AUTO MELEE-->SCENARIO) Then save the vehicle. Exit the game and go to the directory you installed the game to.
In my case this is F:\GAMEZ\GOG\I76\
Then go to the "Interstate 76" folder. From here you need to go into the "Addon" folder. You will see a file with the extension of "VCF". Since I chose the school bus, it's name was like "vxbus01.vcf"
Copy and paste that file 15 times in the "Addon" folder. Rename the copies "vpptXX.vcf"
Replace XX with "01" to "15". Now when you start a trip game you will always have that vehicle!
I figured this out around 10 years ago when I was trying to get a custom car into the Trip mode. When I take screenshots the colors get all messed up, but I've attached one none the less where you can seen Taurus's car and the barn from the first level(early on). I happen to have chosen the bus for this and you can see it aswell (To get into a 3rd person view you press F2)

Cool tip bud

Nice one! That trick is going on the Wikia.

Glad atleast a couple of people tried this! XD

This is cool, I wasn't even aware of that.

Would be so cool if there was a way to build custom missions as well, I couldn't never get around to fully understand how the Editor works, so clunky

As far as I could understand, the editor didn't allow for placing enemy A.I. or "camera movement" events.

Hello, i have finished the game and i wanted to have a custom vehicle for T.R.I.P, however i did what you said in the post but the car did not really show up. I chose Courcheval Cavera.