I just realized I was incorrect and the cigarette lighter is actually on KP-I, not KP-II as I suggested for a power source. A better fused source might be the wire to the window/mirror defrost switch.

You've now got me interested in putting this on my Volvo project list. I just went out and checked my 940s. Neither wire harness has the unused coolant level connector I mentioned near the reservoir like the 960s. The 700s are likely similar without the connector. However, the 900s do have that unused washer level 2-pin connector down by the bottle with the blue/white wire that I mentioned (other wire is black to ground). That wire goes up to the unused instrument cluster dash warning light. You could sever that wire at the cluster connector and splice in a wire to your relay, saving you having to route any wiring through the firewall. If I could set up a float switch to work normally, it would make a nice little project. I'd either sever the wire or push out the pin and simply move it over to that unused coolant level warning light I mentioned. I've already got spare dash bulbs.

I like Art's little float switch that would install through the side of the reservoir. Note that there are vertical supports molded inside the 700/900 reservoirs. You would need to make sure you have internal clearance, also I think drilling through the top the way you do possibly preserves more of the structural integrity of the reservoir walls than the front side. I think the 960 one may install through the bottom where there's a molded stud, so uses a common float switch. Art notes that the 700/900s use the higher pressure green gas cap. One of the first things I did with my turbo after the heater core went and needed to be replaced was switch to a black 240 low pressure rad cap I had on hand. Turbos are a hotter and more hostile environment under the hood and I somehow figured why push my luck with the next heater core. The other car is still the green cap.

Cutting out the radio on your son when the coolant goes low, so funny. You could use the other side of the relay to power the radio to do this, but you'd want to use the radio's KP-I power source otherwise the radio wouldn't work in accessory mode. I'm trying to remember if the 240 instrument clusters have an unused warning light that might be suitable for doing this my original way.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now