electric brakes on a mazda 6

Hi Mick,
If all else fails, read the instructions that come with the brake controller.

Diesel or Petrol?
Never fitted one to a Mazda 6 but often EFI vehicles have a dual section brake light switch.
One half switches the signal for ECU/Auto computer down shift and cruise cancel functions, the other half usually switches the stop lights on.

The brake light section may or may not be conncted to a body module which actually switches the stop lights on. I don't know if conventional or body module controlled. Someone might know.

If it is conventional then it is just a matter of finding the wire which runs to the stop globes for the red wire on the controller.
Main12v battery +ve supply to the controller, has to go through an auto resetting thermo cutout/breaker for safety reasons and apart from the negative to "body/batt neg" the blue wire has to be run from the cab to pin 5 in the towbar base 7 pin connector.

And tap into the wrong wire and possibly damage the ecu
Main power supply wire needs fusing and to be an adequate size, routed safely.
Then the wire down to the back needs to be adequate size, insulated and routed/secured and connected correctly.

I'd get the sparky to do it. That way it's done properly. If you do it and get it wrong....and kill yourself well...that's ok. But it's not ok to put other people in harm's way.

I'm assuming the $500 includes the controller.... for that amount installed I wouldn't consider that expensive for the unit & professional installation.
When choosing a controller consider the Teconsha P3 I have no affiliation with Teconshsa other than being a happy user/consumer