It is replaceable if you can solder.  Its a very small bulb that you can buy
at radio shack.  If you can't solder, its pretty basic for any electronics
repair shop.  If it were mine, I would pull the radio and take it to them.
Labor is pretty high for them to pull it.  On my 99, the dash trim pull off
and the radio slides out.  No tools and its out in a couple of minutes.

It's a pain in the butt.  Most salvage yards have piles of GM radios
stacked to the ceiling.  Call one up and ask if they have one and the
radio code that goes with it.  If they don't have the radio code, it's
a trip to the dealer at the worst.  Usually you can run the vehicle for
two hours, push 1 and 4 (I think) and it will spit out the master code.
Then someone at the dealer calls Delco and gets the code to unlock it.

Really your best option is to just remember that 1 comes before 2 and 4
is between 5 and 6 when it's dark out.