Have to admit, don't really understand this "engine not warming up enough" stuff. Of the ones I've owned (to be fair, one of which was my parents discovery 300tdi when I was a lad, but I used it), all warm up just fine in any weather as long as the thermostat is good. And my buddies 4x - all the engines warm up fine. It's just the cab is still cold. I suppose you guys are living up north a bit and wanting the engine fully up to temp in a short journey?
Viscous fans are just that - viscous - so if the air if cold, they are largely disengaged, enough you can stop the can with your fingers easily enough. (Caution!).
That said, I have now ditched the viscous fan on mine - but only because the fan didn't fit the shroud - so in hot weather, it actually didn't pull enough airflow in traffic. Now replaced with a pair of electric fans that are switched on when the ECU reads warm enough.
I rebuilt my primary heater last year. Better, but still pants. I have high hopes for the secondary heater to supplement it.