So what’s in THAT Factory Fit harness?

GM tweaked their harness designs from year to year, so there are certainly some exceptions to the list below. Fortunately, there is enough consistency to make a few blanket statements about harness design. This is a general guide for which section performs what function:


The big one. As a simplified rule, your dash harness includes the fuse box, ignition and headlight switches and a plug for the rear body wiring. In some applications it will also include a connector to feed the console harness. In rare instances, some vehicles (the ‘55 BelAir, for example) use a one piece dash and engine harness, but that is an exception to the rule.


Nothing too shocking with this one…..starter and ignition. Harnesses modified for HEI will typically see that change happen here. The engine harness also frequently has feeds for the blower motor, oil pressure and temperature sending units.

Front light

The front light harness is a common cause for confusion, because it usually (again, there are almost always exceptions in the automotive world) contains sections of the charging system. In addition to the headlights/parking lights and horn relay, a typical GM front light harness features connections for the external voltage regulator and alternator. Harnesses undergoing modification for an internally regulated alternator will usually see changes in this section.

Rear body

Another slightly tricky one here. Rear body harnesses can change quite a bit depending on the vehicle and year. Some will use a two piece design with an intermediate and rear panel, (this is what would contain the taillights) while others will simply have one harness that includes everything. Our Factory Fit catalog will contain listings for each option when available.

The other harnesses GM used would be smaller, individual sub-pieces used to feed power to a specific option. They would typically be their own standalone part, or would tap into one of the 4 major harnesses listed above. Once you have them sorted, it’s just a matter of plugging it all in!