Just bought a '96 3000GT and former owner put in new battery and says radio needs code put in to work.
This a dealer only thing?

 

pull off the radio bezel (plastic surround, there's just tabs, pop in a flathead screw driver, and pry it off), and there's I think 4 phillips head screws, pull the radio out, call the dealer, and give them the serial number off the radio. Just tell them you lost the code.

 

I got this from another forum - I'll give it a try, first:
"You can get your radio code directly from Mitsubishi for free by calling the following number: 1888 MITSU2006

They'll ask you for some basic car info such as model and trim package as well as year and VIN, but you'll get your code right from them."

 

any dealer can give you the code with the serial number. You have no way of knowing that the radio in there is the one it came with from the factory brand new.

You pop off the black plastic trim piece with a flat head screw driver, and there's 4 phillips head screws holding the radio in. That's it.

 

All done. Radio working now. Dealer gave code - no charge.