We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Jonathon saying
something like:

What is the most economical (cheapest) method of keeping the chill off
the air in a brieze-block built garage? Mine is like a morgue!
Advice will be very much appreciated.

Cheapest? Set fire to the garage.

Realistically: Switch on a fan heater an hour before you go out and
arrange some infra-red spotlamps to shine on areas you work in, like the
benchtop, toolbox and underbonnet, etc.

Much cheaper, if you plan on doing regular cold work in there, is to
build a space heater and flue it out, burning old logs, scrap wood, etc,
in an oil drum.