Most problems with OD are in the relay or solenoid. Yours seems to be somewhat related to shifter action. So here goes...

I suspect your dimming headlights are due to current being temporarily sucked up by a shorted-out OD solenoid wire. That would only last a short while, either till the fuse blows, or till the wire gets knocked into another position due to vibration etc., and isn't shorting any more. More details below.

Re. your post, there are two wires in the shifter housing area related to the overdrive. If just about ready to break, working the shifter could be the last straw.

One wire runs from OD button on shifter up to the the relay, which is under the center vent duct, to the left of the glove box. Not likely your trouble source, IMHO. (access to the relay is via removing the glove box)

The other runs down from the relay, through the shifter box, out a hole at it's front, down into the tranny tunnel, and ultimately to the OD solenoid on the left side of the tranny.

Either wire can break from sufficient flexing, the most common break being in the solenoid wire just after it drops down into the tranny tunnel. It goes to the right side of the driveshaft (surprise!), then forward, and crosses to the left somewhere further up where I've never been able to see it.

It's possible a wire broke in the shifter housing. Pop the plastic cover (2 screws at the front) to get a look. Don't try to completely remove the cover, that's a whole 'nother story. Just swing to one side and look around for broken wires.

The break is often intermittent for a while, with a torn wire doing a make-and-break connection thing. Sometimes it shorts out on local steel, then the OD fuse blows.

If not broken in the shifter housing...
Crawling under the car to feel for the wire in tranny tunnel is pretty doable. Crawling under to fix the broken wire is more of a time-consuming pain, but worth it.

To check the wire, from either side, with a door open, see the shifter box and plan where you'll reach up from below. It's about even with the front of shifter box's plastic cover. From below, slide a hand up the right side of the tranny tunnel and feel around for the wire. You should be able to find it and feel if it's broken or not, etc.

If broken, and if you want to fix it yourself, be sure not to pull the rubber grommet out of the hole at front of shifter box. You'll not easily get it back in unless you have a full shop lift (grommet keeps road noise & water outside). If you do want to pull a broken wire up thru the shifter box's front hole to easily splice on a new wire, resign yourself to patiently poking the new extended wire back out thru the grommet using needle nose pliers. Then of course there's the splicing work to be done below, at the side of the tranny tunnel.

I've fixed that wire on three of our bricks.
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Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).