Amarin's thought about a ground fault at the chassis ground tab behind the ECU is certainly food for more thought.

I'll start out by noting that fuses aren't being blown here so a 12V short directly to ground likely isn't happening. You also say this is a sudden stall out, so the odds of it being charging circuit related aren't overly high, but you have been recently working at the battery so that needs to be kept back of mind. Odds are now higher in my mind that it's a ground fault. Some component is likely getting it's path to ground not through its normal direct to chassis ground path (or only partially if it's corrosion) , but rather backfeeding through another circuit to ground. When the circuit gets activated it is no longer a good ground path and can cause all kinds of odd problems. As an example, when there's a ground fault at a taillight, something odd like touching the brakes when the turn signal is on might suddenly stop the flasher, but that's not the circuit we're interested in here at the moment.

As the headlights are involved here, the ground points at the headlight assemblies (one for each side) are known problem areas for ground faults and could be related to such back feeding when the headlights are on. It would then be something like one lamp beam circuit getting all or part of its ground through the other beam circuit. How that's actually causing your problems here I don't know, but because it's easy to check and a stalk switch related circuit then I'd begin there. Check the chassis ground terminals near the headlight assemblies under the hood latches (loosening/re-tightening just for the heck of it) as well as making sure the headlight connectors are solidly seated, including peeling back the boots and pushing the wires forward (another known problem area as I mentioned before).

Now going back to Amarin's thought: Looking at the GB (UK) headlight wiring diagram again, there is a ground wire directly from the stalk switch, but it's to the ground rail (31/31) underneath the relay tray. That ground rail is shared by all the relays, including the headlight system relays. There's then a ground wire from that rail over to the actual chassis ground (31/10) at the silver metal tab behind the ECU. With so many relays involved then all kinds of weird things could be happening if the ground fault is in that circuit. Now this is for a 1995 940, while I believe yours is a 1991 even though you don't list it in the heading. Normally for this kind of circuit I'd expect it to be consistent throughout the 940 series. In checking a late model 700 diagram just out of curiosity, again all headlight and stalk switch associated grounds go to under the relay tray. If Amarin has access to an early 940 Volvo Green Manual diagram then I'd be interested if it shows different.

Also, the ECU doesn't have it's ground connection at the ground tab (31/10) behind the ECU (as you might logically expect), but rather at the main engine ground point at the fuel rail (31/32) along with things like the AMM. That's another known problem area, so before going further, loosen, clean-up and re-tighten the ground terminals there. That also takes us back to making sure the engine ground from the intake manifold strut is good back to the battery (where you've been working). Hmmm, all the more reason Bill might be right and had it figured right from the beginning?

I shouldn't carry on here until you've checked those easy things out, but having my nose in the wiring diagrams once again then I will, assuming the stallout hasn't gone away.

Looking at it again from the problematic stalk switch, all other paths to ground go through the relays I mentioned and to the relay tray ground rail. Pulling the relays as I suggested one at a time would quickly isolate those ground circuits and might point to the errant ground path without having to start removing all kinds of things to get at other test points. So, step three here, go back and make sure you did as I previously suggested.

Failing all that, you'll need to start opening up things to do further checking, like cleaning up their grounds and doing voltage and resistance measurements between various points. It's a tough call what I'd do next, whether to open up the steering column to test at the stalk switch, or whether to pull the relay tray so you can flip it over to check the grounds and check for a melted socket and loose wires, or whether to take the side panel off to remove the ECU and check the chassis ground tab behind it. I'd say pulling the relay tray is next on your list in terms of known problem areas with high current circuits like headlights possibly causing socket connection and ground problems.

Keep us posted on your progress. We've given you a ton to think about and pursue, so try to go through them in logical order, turning every stone along the way.. I was just thinking that a typical soggy Scottish winter makes it ripe for aggravating ground connections at this timne of year.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now