I never made a custom soundtrack for Excite Truck (and only bought a copy of the game recently, like you, so maybe I will make one now.)
But I did make one for Endless Ocean which I recall being way more complicated that it needed to be. Mainly because, if I remember correctly, you had to pick one and only one track before entering a dive. Then that track would just loop. So the complicated bit came from crudely stringing together different tracks with some PC tool to make one long-ass mp3 that you could then use in-game. Good times. I don't remember all the songs I used, I think most were instrumental, but I do remember sticking Phil Collins "In The Air Tonight" in there somewhere. That was funny.