Start at the battery, and measure for "voltage drops" from there.  
Turn on the interior lights for a load.
With meter lead at the center of the ground post, measure to the battery
cable clamp -- this measures the voltage drop right at the post connection.
It should be <0.1V.  Then measure to metal body of the car in the engine
compartment.  Should be less than 0.2V.  Measure to grounds used in the
interior of the car. You may need some extra wire to extend the probe(s).

Do the same on the 12V side.  You can either measure ground to 12V and then
check that you are within 0.3V of the voltage at the battery.  Ie:  if you
measure 12.2 V at the battery (engine off), then you should measure the same
at the fusebox.  If you read a lower voltage at some point, you can check
the voltage drop across a suspected component such as a relay.  Put one
meter lead on one relay contact;  put the other probe on the other contact;
you should measure nearly zero.

You're really going to need a schematic to troubleshoot effectively.  

But first make damn sure the battery cables are good.  They are the #1 source of
electrical problems as the battery acid attacks them.

A similar problem happened with my cousins Chevy a few months ago. The
battery cable was slightly loose.
Dan

(This account is not used for email.)

clean aand test battery and cables then alt.