Cant find the fuse for radio in a 99 park avenue

Check under back seat

I'd check: (if no luck locating it utilizing the owner's manual - or if you do not have one)
1st - I'd check the Lower Kickpanel near the drivers left foot (Behind plastic cover) and passengers right foot (also behind Plastic cover) - The plastic covers are usually marked "Fues box" or "fuse" in printed letters or embossed. If it doesn't just "pop" out or is attached with screws, then it is not there -- They are usually placed with a plastic cover over them that is marked "fuses" and snaps off... If it is not there - They are also sometimes located inside the glove box behind a plastic cover - and also sometimes located under the drivers side dash (which requires that you contort yourself - from laying in the floorboard - and without a cover when placed there) - and lastly (and least for GM), some are located in the engine compartment (But this is rare on American cars - usually the fuses located in the engine compartment are for engine accessories and not for the radio) ...
2nd - alot of GM radios have an "extra" fuse located in the back of the radio itself- thus, you must remove the radio in order to check it, but first I'd locate and check the primary (fuse box) mounted fuse for the radio and ensure 100% that it is good....Best of luck...You should find it if you look in those places...

Have you reviewed your Owners Manual for the Location of the Fuse?
There are Fuse Blocks in the Passenger Compartment and Under the Hood of almost all recent GM Vehicles.

The Owners Manual typically is REQUIRED READING fo ALL CAR AND TRUCK OWNERS. Start there first!