Hi,

It is most likely that you either popped a fusible link during battery installation or if you had the positive terminal apart one of the power feeds fell off and went unnoticed. Both of these are extremely common occurrences and result in little to no power in the vehicle.

The code for the radio has no impact on the power in the vehicle or the ability to start the vehicle. It -only- prevents radio operation; it is a theft deterrent for the radio itself, but has no interaction with the rest of the vehicle.
If the replacement radio is aftermarket, then you have nothing to worry about with regard to the code. If the replacement radio is another Mitsubishi radio, I can provide a new unlock code for you if you provide me with the serial number from the top of the new radio.

It sounds like you just either popped a fusible link or dropped a power feed though. Let me know what you find.