There are plenty of 700's in junkyards now, and swapping the detector is a pretty simple job. Just remove the coin tray above the ashtray and pop out the sensor. It's a big red can in the front left of your fusebox. They do sometimes fail, though the most common problem I've seen is that the brake lights quit working. After changing both headlight bulbs (9004 halogens, if they're stock, by the way) so that you KNOW they're identical, I would try replacing the bulb-out sensor. Also check headlight ground wires for clean connections.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total