Mine is under the wooden console shifter cover. Take out the ash tray, unscrew the two screws in the ash tray frame and remove the frame...THEN lift up the wooden shifter cover console thing (with the window switches in it). Mine has an orange wire and a yellow wire under there running next to the shifter going to the radio, both with inline fuses. I can't remember which color was which, but one was constant and the other was illumination (+12v with the parking lights on) but the constant had a much smaller fuse in it...I think 0.5 amps. The illumination wire had a 2 amp fuse.
Hope your's is similar. Good luck.
-Geoff

EDIT - For a redneck current tester, pull the bulb out of the dome light and connect a hot wire to one side and a ground to the other. But really...I got a multimeter from sears for $20 and use it all the time. I should have gotten a better one. If you think you're going to do any automotive electrical work after the stereo (and you probably will on a 20+ year old car), the multimeter is worth it.

(Also edited because I mistakenly said the illumination wire was key-on power)

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now