Okay i know that you know about this stuff so hopefully you know what i am on about.

I am thinking of adding a second head unit to my car due to my girlfriends refusal to carry an empeg around so she want a normal stero in the car.

I have no room for a second din but i do have room for another radio so the plan is to get a normal face off stereo and putting the main body stuffaway some where in the car and then have the face mounted in the dash.

Most face off stereos have about ten to fourteen connectors on the back, how would i see the longest cable i could get and what length do you think i could get and what would be the best cable etc.

Hi.

Though I am not GOD - hmm, PCA - I still would like to add my opinion:
The length and type of cable you would be able to use is depending on the type of interface the headunit in question is using on its particular faceplate connection. I know that both multiwire serial connection (like one for display issues and one for buttons) as well as parallel connections with various connections speed exist. The higher the speed, the shorter the cable has to be.
In general, you might be best of with a relatively old headunit which doesn't have a graphic enabled display (i.e. has a text/segments only sort of display), because those don't need a high interface speed to the faceplate and thus tend to have a slower interface which in turn would allow longer wires.
Also, the higher the interface speed goes, the more important it will be to match the expected impedance on the wire, which is rather hard to find out IMHO.

Summerizing this, it is a tough task you are challenging here. The longer the cable needs to be, the tougher the task gets.

cu,
sven

so grab an old unit and play around and see what happens