Shoulder Belt in Rear of '89 F250

I recently purchased a 1989 F250 extended cab and it has rear lap belts only in it. I need to use a booster seat for my 3 year old which requires a shoulder belt.
I've called Ford and a number of dealerships and they can't give me any suggestions on where to mount an aftermarket shoulder belt.
Does anyone have any ideas on my problem? Perhaps a safe mounting point or where to find such information?
Thanks!
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diego wrote:

Did you look behind the trim where the shoulder belt mounting points should be? It became Federal law about then for rear shoulder belts mounts to be installed, but not the belts themselves.
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Hi I once took off the trim panel to look to see if there was an obvious mounting location but I didn't see anything in the area where I would expect to see one. I guess this particular model was before that regulation came into effect. I am tempted to bolt in a sturdy metal plate in the sheet metal behind the trim and then attach the belt to this. I am concerned though that this might not be safe.
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