Steve,
No, you won't lose power to the stereo necessitating code unless you also disconnect the other plugs on the back side. You should be able to get the C3 plug hooked up w/o doing so.
However, it'll be tight trying to fish it down/behind the radio frame w/ everything still connected (unless you have a helper to hold the radio. So, you may want to assemble the plug and wires first and feed them up from the bottom to the cavity so you can just pull the plug and connect while leaving everything else attached.
Couple of tips:
use a screwdriver in the head of the keys to pull with - it takes a fair amount of effort to pull - the key heads are sharp and could cut your hand. Effort to pull depends on how often radio was pulled and re-insterted previously.
Don't put the keys in until you want to pull the radio out b/c in order to release them, you must pull the h/u out push the blue tabs on each side to free them.
You should get your radio code. Some dealers will give them for free (most will now a days in my experience w/ proof of ownership, vin # and or radio serial #) or you can post your radio serial # in sticky thread of Boxster section over on Renntech.org and request it from Loren.
Good luck
