Just the xxx_mods.rpf that I'm using?
The mods are meant for the McLaren P1 mod from GTA5-Mods. It originally replaced the T20 but I edited it (both the car model and tuning models) to be "mp1". The car works fine in game, everything in carvariations and carcols.meta references the "mp1" parts within that archive. Every stat tuning part and wheels work, but for some reason these parts just don't appear.
Like I said I installed the Lexus the exact same way and every part shows up.
EDIT: Also, I've set up a new americandlc.rpf according to your guide. I've done everything right as far as I'm aware, but you didn't seem to explain packaging it back into the game. Every .rpf file in a DLC container needs to be listed in content.xml, right? Because I'm trying to figure it out myself and it's not reading the new files.