RE: reducing the trade-in value by installing radios...
I used to worry about that, but after trading in a few cars with radio installs (that were removed before trade-in), I'm convinced that unless you leave gaping holes or dangling wires, (things that look ugly or raise questions) the radio install is the least of your worries when it comes to resale value.
After a couple of years, the general wear and tear of a car makes a few holes here and there a non-issue. Roof mounts? cover them with a cap. Dash mounts, fill/cover the holes and no one will bat an eye. Extra power wires? Dress them up or advertise it as "wired for a bumpin' stereo install!" (and charge more
If you do a crappy install, sure, that will lower the value because of the damage that will need to be repaired to make the car sellable. But if you do a decent install, to bend over backwards in the name of "saving the resale value" for 99% of the cars on the road is just silly (IMO).