Not sure whats really causing it, i can try at some point to download a newer version of the game through steamdb with the needed suspension and add it to this mod to see if that works (iirc the suspension is either from 0.9 or 0.14 and the grand marshal was revamped in 0.17)
Minor update 2, Changed new engine sounds to the barstows i6 sound (does not sound as bad as you think) due to sound complications with how old this mod is, Corrected "Volts" needle. Update can be found here. EDIT: i am aware of the absolutely atrocious suspension, unfortunately theres not much i can do about it I believe, i may try and find a newer suspension to use and see if that fixes it, but theres not much else i can do sadly.
Surprisingly the newer suspension actually works better, I've added a new file to my original post, let me know if you think it works better or not. Also fixed a no texture badge appearing on the regular Grand Marshal if a regular Grand Marshal is spawned after spawning this mod. File located here.
Some spontaneous updating, -Added interior sounds, -Added/fixed siren, -Revised and added some configs to better fit with the newer game configs, -Modified rear concealed flashers to not clip through rear window. New file here.
I would like to make it openable but i still have no idea how the openable system works so i cant add them for now --- Post updated --- For some reason adding interior sounds also made the interior muffling sound break so I've had to rollback the interior sound change indefinitely unless I can find a way to fix it, new file on my original post.