Engine Block Heater Question.

[nmikes]

Might be relocating to Minnesota so was looking into the engine block heater. Does anyone know about the OEM one? Does it heat the engine by cycling fluid through it, or is it a chincy magnetic heating pad you stick on the bottom of the oil pan. My dealer here in Seattle is great, but there just isn't alot of call for this item and so they've never seen one. Just wondering if the OEM unit is a really good one or if I should be looking elsewhere. The dealer is wanting also $250 to include parts and installation... seems a bit high for an item that lists for $20 bucks on the hummer web site.