Under certain conditions, like when power is disconnected or the radio is removed, the OEM Delco radio in 1990 & 1991 model Reattas will sometimes display 'LOC' in the digital readout and no functions on the radio will work. This is due to a security feature built into the radios.
Here is the solution:
1. Turn the radio on so it will display LOC.
2. Remove the fuse box cover on the passenger side of the center console. There should be a radio fuse there (the inside of the cover shows the fuse numbers, values and function).
3. Pull the radio fuse for several seconds. Reinsert the fuse and check radio. If still showing Loc, repeat up to several times, as apparently it sometimes takes repeated interruption of power to clear it. (sometimes 5-10 times)
That should eventually clear a LOC condition on GM radios of this era.