If you've got a MkIIa unit and a MkII sled and it won't power up, the problem most likely lies in the constant +12V wire/connections.

A MkII can be completely powered from the switched +12V line alone, but the MKIIa uses the switched +12V ONLY for telling it to turn on/off while drawing all power from the constant +12V line. See this FA Question

Even if you can't fix it yourself, this might help speed up troubleshooting for whoever that will do it...

/Michael