Just after starting what would cause the blower motor to slow down and headlights to dim?
Alternator?

 

you got it...

 

the grid heaters. once your truck gets to opperating temp or exceeds 20-25mph the grid heaters quit cycling. these heaters heat up the incoming air to help get the truck warmed up faster, cranked up faster and idle better when cold out. they pull massive ammounts of current and you can watch the volt meter drop a couple of volts or so as they cycle on and off.

 

Just one of their quirks... enjoy it.

 

that was my other thought. just one of the quirks.