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For basic troubleshooting help, see: http://www.sw-em.com/electrical_troubleshooting_notes.htm , but you have given us very little with which to help you...remote troubleshooting, especially of intermittent problems, without more info is just about impossible...and a theoretical list of what all COULD the problems is semi-useless...so I suggest you do some initial checking and get back to us with more details (are instument and front marker lights functional, while taillights are not etc.? Under what circumstances where they intermittent?)...you may even in the process find and remedy the problem...

Circuit diagrams are available in your owners manuals and here: http://www.sw-em.com/122S%20Wiring%20Diagram.jpg

General: (Refer to wiring diagram) Power for any lamp in the taillight assy comes from switch up front (lightswitch, brake light switch, dir. signal switch), by way of six conductor connector first under dash (at A pillar), then three more six conductor connectors in trunk. You could simply disconnect the main one in the trunk and check for voltage with a test light there (if power is present, problem is downstream, if not, back up front)...then powers fixture in taillight assy. ALL connections in current path MUST be clean, snug, and free of corrosion! Alone at the taillight assy there are no less than SIX places where opens or marginal electrical connections can exist! See: http://www.sw-em.com/Amazon_Rear_Light_Fixture_Restoration.htm

If you have the taillight assy open/apart, you should consider a refurb per article at that time!

Good Hunting!