Parasitic battery drain - 2001 Spyder GS

Hi:  I've recently had two batteries put in my car and they've both passed
the drain test.  My starter has been replaced and I've been told that my
alternator is charging okay.  My belts might be loose though.  I don't know if
it's the serpentine belt or the timing belt or some other belt.  The car has
just over 100k miles on it.

After the first battery change, the car door locks stopped locking with the
switch and the doors chimed when ajar, with or without the key in the
ignition, as if the headlights had been left on or something.  

When I asked about the drain, nothing was detected because it was
intermittent.  I understand about checking fuses and circuits but I can't do
this on my own.  I need to know where I can take this car to get it diagnosed
and fixed.  I don't want to take it to the dealer but the other shops can't
seem to detect the drain, if there is one.

The other thing that might be relevant is that the left turn signal switch is
broken and I have to manually return the arm to neutral.  Also, the radio
volume is broken.  This has been going on for a while though, with no drain.  
This battery died in three months.  Now, I'm afraid that it will keep dying.  

Can I take this to a regular shop or do I need a master mechanic?  This
testing seems laborious and complicated.  Any advice would be greatly
appreciated.  Thanks.