As Moki said.
I wired mine using a 12 volt automotive relay. I think I bought a Narva from Dick Smith for about $8? It doesn't come with wiring instructions, but the pin-out on the back is industry standard and google takes care of that. I used the instructions on this site:
http://www.canyonchasers.com/shop/generic/relay.php
All you need to do it (4 pins to connect on the relay):
1. Run a dedicated connection from the battery to the main feed of the relay
2. Tap into your brake light (or any other circuit that switches on when the ignition key is on) - feed that into the relay on the pin that triggers the relay (ie. completes the connection from the main battery feed
3. Wire in the ground on the relay
4. Wire in the switched circuit on the relay to the hot grips.
Voila - hot grips you can never forget to turn off.
I'm pretty dumb when it comes to electrical things... and even I could figure it out. The instructions above are really good. One flat battery will convince you it's worth it :-)
To be fair - a mechanic may not have auto-electrical training hence the initial comment. Also - it could be costly if your bike makes it had to tap into an ignition source - as labour is involved in removing panels / parts.
The cost of parts themselves should be pretty cheap. As noted the relay is the most expensive part - followed by a few connectors and some wire.
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