help needed for factory tach

[evilzee28]

I have a '69 Camaro with factory tach and cannot get it to work with
either stock points setup or MSD6 using the MSD tach output. I cannot
find the mythical 'filter' that MSD refer to so guess it's gone
already. Needle goes to 1000rpm with ign on and engine not running,
stays there even when driving spare dizzy with electric drill to
simulate running engine.Turn sidelights on, revs drop to 500rpm. Turn
headlights on, rpm drops to zero like voltage drop is affecting it.

Here's the thing: car has had a repro wiring loom and I have already
found two errors which caused lighting problems so I'm wondering if the
dash part is also wrong and causing this issue. Anyone able to tell me
for definite which connection on the tach is which? Multi plug
connector goes onto a 3 prong unit arranged like an inverted 'U'. One
is 12v power, one is earth and the remaining one is pulse but which is
which? Does this sound like it could be the problem?
Thanks, any help appreciated.......Nige