[paulgato]

Oldtamiyaphile, thanks for the idea of putting a step-up voltage regulator unit before the headlights to keep the voltage constant there. I hadn't thought of that. It wouldn't even need to be a sophisticated unit. I could even use one (or two?) of those cheap in-car laptop chargers that output 15v.

[EDIT: ...but I'll try the simple and cheap option of fitting a pair of relays first so I can feed the headlights from a pair of short, thick cables with inline fuses direct from the battery. That will give the headlights an extra volt, which may be all I need.]

As for the PWM controller idea, that 10A PWM unit should be suitable for the heater blower but you might need to bypass it with a switch for the highest speed. At 12.5v my blower motor takes about 16A at full speed, 8A at speed 3, 4A at speed 2 and 2A at speed 1, and at 14.5v those motor loads will be proportionately higher of course, so 19A at full speed and 9.5A at speed 3. I got a 15A PWM so I would have to do the same thing and bypass it for speed 4.

(Frustratingly, I also bought a 30A PWM from China but then discovered it has a common positive rather than a common negative, so not good for use in a car with negative earth! My mistake: I should have asked before buying it. The 15A one should be fine.)

...but why are you using a PWM for controlling DRL's? Are the DRL's on your car just the headlights fed via a resistor?