I've installed block heaters in three Red Blocks. All were California market cars. I believe back in the day, Canadian market cars may have come with them standard. I'd be surprised if that still held true, as Volvos are now a semi-luxury car, and the dealers don't miss many tricks to extract a bit more cash from each sale.

I installed both 240's in the rear-most frost plug hole below the exhaust manifold. This was the position called for in the heater instructions. Installation wasn't very difficult, as I removed the downpipe from the manifold for extra working room. After 22 years in the 83 and 4yrs in the 81, no problems with the power cord - I routed it to stay as far as I could from the exhaust.

The instructions for the heater for the 940 called for the unit to be placed in the front frost plug hole. This was a bear! The PS pump, and the heater pipe to the back of the water pump made access very difficult - but eventually it was in and seated without leaks. (The frost plug holes in the B230's are metric and different from the B21/23.) Doing it again, I would use the rearmost plug hole.

I examined one of those rad hose heaters in the PnP, and didn't like the looks of it: a) because of the restriction in diameter, b) its small size didn't inspire confidence in its heating power, c) positioned where it is, it just didn't look like it would heat the engine's insides very well.

IMHO, those magnetic stick-on heaters are all but worthless. Five of their six surfaces are not touching the block, and the one that is, is sitting on a rough and uneven surface. Adios, heat transfer.

The block heater that inserts its heating element right into the engine's coolant passages is the way to go. Aside from the personal comfort factor of having the heater blow warm right away, there's the reduction in engine wear from much faster warmup, and the safety of having windows defrosted right from the start.

Cost: 2 years ago I had a shop supply and install a block heater in a Dodge Caravan (FWD, V6). Total was $140. Around here you can get the heater kit (includes power cord) at most parts stores for about $45.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)