Any ideas on trouble shooting an electrical problem.

The light over the speedometer is not working.  The van is an 85 vanagon
with a tach & speedometer.  All seemed to work until I pulled the cover
off to get to the brake fluid.  Now the light over the speedometer does
not work.  The light over the tach works most of the time.  When  it
does not work I found I can turn on the emergency flashers and then the
tach light works.

Might it be the bulb?  If so, how does one replace this little sucker?
Any other thoughts or suggestions will be gratefully accepted.
Mechanical problems I can handle but electrical problems I find most frustrating.

Many many thanks

Brad
85 vanagon