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Hi again,
It sounds like the check engine light isn't your priority today. Let's work on the backup lights. I use them to see where I'm going backing down a dark driveway. I found out others use them to look for a parking space that is about to free up.
I think Laine's experience with corrosion in the trunk will be the most likely place you wind up fixing this. She wasn't talking about rust, like iron rust, but copper and tin - more on a microscopic level. If there's a water leak it can get visible fast, but it doesn't take much at all to stop 12V electricity.
Sunday, when my daughter came to visit, I spent an hour and a half fixing a taillight problem with the connector Laine is talking about. The flexible circuit board in the taillight assembly wraps around a thin tongue of plastic to support it where the white L-shaped plug connects.
That plug was designed to squeeze the edge of a rigid circuit card 1/16" thick to make good contact to the copper. But when the contact is not so good, because of a tiny bit of corrosion, it heats up and melts the plastic tongue underneath, soft enough for the plug's contacts to squeeze it thin -- too thin to afford good contact. Here's a close-up photo of the edge of that tongue with the flexible circuit wrapped around. It appears to be badly damaged:
In my daughter's case, I had to give up on cleaning the connector or circuit contact. It wouldn't solve the problem of the melted plastic. It sounds like Laine solved that problem in her car by adding some copper foil to build up the thickness. That takes skill, and my daughter lives some distance away, so I bit the bullet and wired around the plug, soldering in a wire between the cabling and the copper foil near the lamp socket. I was kind of sick about doing that, because her taillamps are relatively new genuine Volvo (Valeo) pieces of uh, expensive artwork.
In your case, I gather someone has already replaced parts to try to fix the backup lights. We can't see or feel 12 volt electricity, so when replacing suspect parts doesn't do it, the circuit has to be traced. I remind myself of this when I can't seem to make a string of Christmas lights work. But this is so much easier than a string of lights, in a 240 Volvo, once you get a simple tracing tool, a test light.
You can find a 12-volt test light in anyone's automotive section, for instance, WalMart. Maybe it costs $3. If not, it should. The purpose is to see the presence of electricity. Buy the cheapest one because it will be simple to use: clip one end to a ground, something metal on the car body, and use the probe to check for voltage, as Ryan suggested, using a diagram.
Start at the source (this gives you confidence the test light works). The source is the fuse panel. Then go to the next conveniently accessible place. For a backup light circuit, that is not the shifter, or its plug under the rug. Go there, only when you don't have power on the contact in the left taillight's L-connector at the black wire. You just trace it back to where the electricity stops.
Here are those diagrams. The wire colors are helpful, as it is to know 'SB' is "Swedish Black". :)
Let us know how you make out.
--
Art Benstein near Baltimore
A backward poet writes inverse.
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