After market head unit install 1998 2door
 

There is an after market head unit in my 1998 2door 4wd. I replaced the battery yesterday, but now the radio says "No Code" so do I need to have a code entered in the head unit?

check the glove box the code may be with the owners manual or a stop at your local dealership should be able to fix up the code for you

If you decide to go aftermarket Pretty much any aftermarket stereo head will work
You just need a harness kit and a dash kit with it
Your local auto supply should have them but here are some examples

Are you sure that the radio is aftermarket? GM used a system called Theftlock where the user could enable a code to lock the radio to prevent theft. I believe there is a system to get the code for a GM radio, but you'll have to Google it. It may be easier to just get a new one.

If the radio is indeed aftermarket and employes a similar PIN type code, then unless the previous owner marked the code down somewhere you may be SOL. Like Rexazz said, check the owners manual and any related papers in the truck. Hopefully you'll be lucky and find it written down somewhere.

It's an aftermarket Kenwood already in the dash. I'll tinker with it tonight and look around for the code.

I misread
Thought it was the stock head

cant say I have seen an aftermarket that needs a pin in a long time
I thought everything was detachable face now

I vaguely remember some old Kenwood decks had that "feature". Google the model number and PIN and see if you can find a way to reset it. Because it's an anti-theft "feature" I suspect that there isn't an easy way to reset it. You could try some generic pins like 0000 or 1234. Something like that.

Got it figured out. Had to use the remote to scroll through the source to get to the tuner.