yes and NO
yes you can build one,
but according to posts on HoFo ( Howards Forums) the original OS rom, would shut the mobely off after detecting a constant 12v for a measured amount of time
after all when the engine is on and the vehicle is moving the voltage varies some what from 13+ to 14+
so home built power supplies were built with timing circuits to vary the voltage
the new version OS rom with timer shut off (never shut down) may not need this feature, but I would not dismiss the $35 power supply as a rip off
The Mfg deserve some profit, and the eBay seller is there to make some $$
How much do you value your time ?, the Mfg and seller value their time also
You can build it for $15 in parts but you wouldn't sell out for $15
I could build one, I chose to buy one
