A test assembly for testing a preassembled wiring harness for location of mounting brackets at spaced points on the harness assembly and for evaluating terminals of the wiring harness includes a faceplate with a plurality of slots formed across the full planar extent thereof to accommodate a variety of different holder and detecting device layouts corresponding to a variety of types of wiring harness configurations. A separate faceplate is supported on each of a plurality of modular units joined together to accept different length wiring harness configurations. Standard length cables are connected to each of the holders and detecting devices. The cables have a connector wired thereto which fits through the slots to provide access from the faceplate to the interface cards without splicing into the wiring system. The modular frame units include a standardized mounting arrangement for interface boards or cards that include circuit components for simulating different modes of wiring harness use to be evaluated by a wiring harness tester control. The modular frame system includes a support platform thereon configured to releasably secure the harness tester in a fixed location with respect to the interface board components.