A dealer will find every excuse to cut down your trade-in value... and that's a deduction from their already ridiculously low offer.

Mods don't mean jack as far as the dealers are concerned... it only matters once the vehicle is in their possession. Then, they turn around and ask for a few thousand more than Blue Book (which is high to begin with) and cite the mods to potential buyers as the reasons for the markup.

When I sold my VW I did much better returning it to as stock as I could and then selling the mods separately. The offers I got while the car had mods weren't even ridiculous... they were moronic.