Uh, why? Toyota didn't expect the bed to be grounded; that's why there's a separate W-B wire (ground) pulled all the way from the front back to the taillights (same with the stop lights and dome lights, etc.) On mine (a '94) the bed is definitely NOT at ground. (This could be a model year thing. I expect that older, more exposed, trucks may end up with the bed "near" ground as the rubber vibration insulators wear out.)

 

Grounding must be a model-specific thing, Scope.
Agree 110% about having dedicated ground wire (white wire with black stripe).
On the 1986 4Runner, I do not see a ground to the frame. However, there is a dedicated ground wire for circuits in the back and that ground wire is connected to body ground behind the driver side kick panel, and the rear body. I strongly believe in grounding metal parts that could develop a high static charge for safety. Must be same reasoning why Toyota grounded the body on our first-gen.

In O.P.'s case, I suggest that IF impossible for him to physically trace the ground wire to be absolutely sure the ground circuit is complete, to use the bed metal as ground path... Rather than leaving the ground path a mystery