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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I have 5 kids 8 and under. Believe me, I can completely relate to your problem. May I suggest that far easier than what you're planning to do would be to buy a new child seat. They make boosters with 5 point harnesses that will fasten to your truck using the seatbelt. That way you get the safety of the harness without modifying your truck. Sure the seat will cost a little bit, but probably will be less than the time/trouble/parts that you'll need to do what you're proposing.
CJB
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Hi, I just spent some time talking with a local Ford Dealership and it appears that without a factory installed strongpoint behind the trim they won't offer me any options for fear of liability issues. I then spoke with an auto upholstery shop that does retrofit seat belts and they told me that they've tried in my year of pickup and haven't had any luch finding an adequate mounting point.
I then spoke with a local car seat retailer and they said that the largest 5 point harness type childseat available in Canada is one that goes up to 47 pounds. I did find a "youth" seat on the Britax brand website which would be good up to 80 lbs but for whatever reason not legal here in Canada.
Looks like i'm going to go with the seat to 47 lbs and then once my guy is too big figure somehting else out.
Thanks
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