If nothing at all works (not even lights or horns), the first place I would check is the battery cables at the battery. Very common to get corrosion between the clamp and post that is more or less invisible but won't conduct enough to power the car. If either of the cables has one of those clamps that also clamp to the wire, just replace the entire cable with the proper one.
Otherwise, turn on the headlights and start checking for power with a test light or DMM, starting on top of the battery posts and working outward from there. Depending on what year TR6, there may be a junction block in the hot battery cable, which is also a common trouble spot.