Assume it's a BJ8, not s Sprite? My BJ8 came with a 'stock' radio, a BMC. which was likely dealer-installed (based on the shabby cuts on the console vinyl). You'll need to remove the console, and the shifter boot and knob, but it's otherwise a bunch of screws (4 machine screws under the fascia/dash, and sheet metal screws elsewhere). The four knobs--tuner, volume, 'tone' (hah), etc.--just pull off, and there are two thin locknuts holding the radio to its faceplate (5/8" IIRC--you'll need a deep socket if they're not finger tight). Behind the radio, there are is a bracket on the console that hooks a couple bolts on the radio; with the radio otherwise loose the console can be tilted to free the radio (you have to work it, and the speaker bracket, around the heater's heat exchanger). Hopefully, there are connectors on the power lead (probably fused)--I don't recall if there's a ground lead (probably--you need one because there isn't a robust ground otherwise). The antenna cable has a 'banana' connector that should just pull out.
If it's aftermarket, all bets are off (but should be similar).