Trouble reading wiring diagram

So, I installed my Dorman seat heaters on the driver's side by tapping into the harness that runs under the door sill... now, the Dorman dashboard switch only indicates operation on High setting, and goes dark on Low setting, and the heat output isn't stellar.

I tapped into a black wire that APPEARS to be a taillight line. I'm concerned that:
1) This line is not able to support the demand
2) There's another concern that I don't know how to troubleshoot that will require ripping out the whole kit, which would really tick me off.

I'm thinking a good first step would be assuring that the circuit I tapped really is wheat I thought it was. My skills at reading the wiring schematic are... not so good.

Any suggestions?

 

I have a Haynes manual, but a copy can be seen here:

http://we0h.us/Prius/Service-Manual/Gen-III-ZVW30/2010-Toyota-Prius-Electrical-Wiring-Diagrams.pdf

Oh, I should have mentioned- I'm having trouble even finding the circuit I tapped into on these diagrams!