Don't give up yet.
I don't believe the main engine controls have squat to do with lights.

First, I don't have the correct wiring diagram for your car and model.
You can search for the correct wiring diagram on the web. It is essential to
diagnose electrical issues. Volvo has very good manuals and you can find it
usually for free. GET IT! It also shows you pictorials of what the part looks
like and where it is, so you can find it.

Second, the lights seem to change less year by year than other items so here
is my take working with a 95 850 diagram:

Headlights Low beam intermittant on one side: Swap out Main headlight relay
it has the bulb out sensor circuit for each low beam.
The ground is common to both low beams so if one low beam is on, the problem is in the +12VDC going away.
The main headlight relay is in the main fuse block in the 95 850.

Tail Lights/brake light: Looks like a Bulb out sensor again. In the 850 its buried low down under the Left tail light.

It's odd that both Bulb Out sensors would fail at once, these are not like the older sensors that required soldering, so forget that. I'm curious if your car had any welding done on it. Ask others on Brickboard if leaving the battery connected while welding can cause damage to the electronics.

Both of these items might be short money at the local boneyard.

The climate control is something else, I don't know what is supposed to happen
and others can chime in on a possible bad sensor that is calling the motor to come on or whatever.

Post back with stories of DIY joy.

Bill

You do have a wiring issue. The first place I would check for the rear lights is the harness at the lift gate hinge. You will have to cut open the cover to see the wires, have lots of electrical tape ready. Before cutting, you might want to probe the brake light with a meter.

There is a ground by the right front headlight, check it for corrosion.

The cabin blower running is a short that is delivering 12V to the ground and thereby supplying the blower. The ECU only operates the blower after the AC has been on and the car shut off. It could be a fault of the relay for the blower.

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Keeping it running is better than buying new

double click

The brake light bulbs are held in place by "holders". They quarter turn and lock into place but the electric contact is quite small. A tab of metal which is rounded (bent) contacts an edge. Scorching effects these.
One of my holders was melted and slightly warped and I had to jiggle it whilst a weight on the brake pedal kept the circuit hot. Even after cleaning I had to use a rubber band to lean it over to make contact. Afterwards, it settled into place.

The position lamps (the round bulbs at the top of the lower section, no?) are held in with smaller versions of the same holder.
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Our 3rd V70

I don’t know a lot about the ECM problems but I doubt it. Certainly not the lights. The climate control issue could be a short, not sure about that one.

The light issues sound similar to some issues I dealt with years ago with my 90 240. I’d start by thoroughly cleaning the grounds of all corrosion using electrical cleaner and a fine grade of sandpaper. Also remove the affected bulbs and clean the contacts as well. Then move onto the fusebox, I’m not sure of the best method, but clean the contacts there and put in new fuses for the affected circuits. This shouldn’t cost much more than your time and with luck might fix your problems. Worth a shot...
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Will I buy another Volvo??? We'll see....