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I've got a 4 point RPM harness. The bottom two straps are hooked into CAMS spec eye bolts which are screwed into the seatbelt bolt hole down behind the seat on the outside of the car, and the other into the seat belt rail hole on the inside. The rears are hooked into two more eye bolts which are drilled through the Brown Davis roll bar, through the parcel shelf and secured underneath with CAMS spec backing plates with captive nuts.

Do not fit a harness unless you have at least a half cage. No point fitting a device designed so you can't duck unless you have the cage to stop you from having to duck. Also, you can purchase the brown davis with mountings built in for the eye bolts.