Gosh that is a real 1970s trip into the past (with some bits from 1998) but much of that stuff is still current.
some more hints more for older stuff
http://www.aeroelectric.com/Installation_Data/Jerry_Gordon%27s_TecnNote.pdf
I have fixed the first link. That’s actually a good site. The one in post #5 is bigger and has a fair bit of what I have.
Some years ago Garmin lawyers were trowling the internet and demanding removal of IMs but they seem to have stopped now. And Garmin MMs have rarely appeared; the GNS430 one is ok for dumb repair work but has no circuit diagrams.
Oooooops!!!
I totally missed the YEAR in these posts!!!
That’s because the earlier posts in this thread are from 2013.
I normally robot-download any site I find and put the content on my private site 
These open sites don’t last long because e.g. Garmin lawyers try to shut down any site carrying stuff reserved for dealers e.g. installation manuals.
The first 3 links are dead.
Peter, there are plenty of installation/maintenance/overhaul manuals there - that's where I found many of those I needed.
I have a feeling (without downloading the lot) that most of them are pilot guides, rather than installation or maintenance manuals.
Here is another collection of avionics manuals,
and here are Continental and Lycoming 4-cyl engine ones.
