I think I’ve isolated the problem. I covered up pin 2 on the connector (amp remote) and plugged the unit in. It powered up correctly and begun playing some tracks

Wow, that's good diagnostic work. Sounds like you could have handled installing the thing yourself. And perhaps done it right in the first place.

Yeah, do what Hugo said. Make sure that the amplifier's main power is connected directly to your car battery (via a fuse of course), and that the only thing the Amp Remote line is connected to is the amp's remote-turn-on lead. Check and make sure the Amp Remote line isn't grounding somewhere accidentally (kinked under the upholstery and rubbing against a metal edge, for instance).

If these so-called "professional installers" tried to power your amp off of the amp-remote line, we'd like to know their name so that we can can recommend avoiding them in the future.

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Tony Fabris