It's complicated. I may not be more intelligent than you... but I'll try to answer as best I can.
The firing system is powder active and is very dangerous in modern seat belts.
Many older trucks, which have been in accidents, have had the firing igniter's triggered.
The really old trucks, don't even have the firing system.
Which is why they will not return properly and fail to lock up correctly, when they get full of dirt from years of dusty roads.
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The powder is corrosive, after it is triggered, if you have the type with firing systems. Do not dispose of un-fired seat belt re-tractors into the trash or fire!
They are hazmat. You can become badly injured messing with them.
My Chevy truck and my 2009 Honda have seat belt webbing tied to stock hard mounts and stock belt webbing.
I grabbed extra hard mounts from various vehicles, so I have a supply of mounting options.
The hard mounts usually are stamped a rolled edge or plastic protection molded, so the straps, when folded and doubled over, through the small holes on the mounts and the belts, resist cutting.
All re-tractors are removed from every used vehicle I own. That's my fix. It is not my fault if you try this and you get hurt or the person in your passenger seat is hurt while driving. If you have any doubts, DON'T DO IT.
There is nothing in the DOT Regulations, from what I have read, which states you MUST replace a seat belt trigger system or the vehicle cannot be driven on the road.
If memory serves me correctly it says (paraphrasing): Seat belts must be installed for every passenger and should be worn, especially where state regulations require you do so. When the triggers fail, it's on the manufacturer. So I take out the part which usually fails or may have already been triggered and cannot operate at all.
Presuming you do not have the Type "B" clasp... mentioned above, how many of you have ever actually disassembled and inspected the firing systems in your belts and verified they can even fire?
Modern Aircraft seat belts do not usually have re-tractors. Crashing in a plane, requires you return to your seat and fasten your seat belts. No re-tractors. They slip by hand because it is the most reliable way of tightening.
As far as I have researched, when I was training to fly, as long as the seat belts are present, they are in order. There is no seat belt re-tractor inspection, to the best of my knowledge, when registering or having a car inspected for emissions.
If the fabric is rotten, buy the long belt materials and have the strap stitched or tie some knots. Any rigging company which makes certified rigging slings can re-sew your straps, if you ask nicely.
They might even use Kevlar, which is much stronger than nylon seat belt thread.
Unless I have mis-read/remembered the regulations, you may choose to mount the aftermarket units (or tie some bowline knots) re-using your old extra long webbing, so long as it is supple and flex-able, not rotten and hard.
The strap from the coil, inside the re-tractor may have very little sun exposure. If you cut the section with hair spray and vomit off at the seat belt clasp across your waste and shoulder, and have the skill to tie a proper MICRO bowline to the buckle hasp, you're probably going to survive, as best as anyone in a plane crash. Check your knots often. Pull them tight with vice grips, after wetting the nylon.
I used to hang from the roof of stadiums and coliseum's, off the same knots and straps rated for the same load. My rigging harnesses had seat belt straps for the leggings. I have crawled under structural steel out to the middle of the stadiums upside down and strapped in, when I got where I was required.
If you are afraid of tying your own webbing, don't do it.
If you are afraid of crawling upside down, into the middle of a stadium under an i-beam, don't do that either.
The bowline is the strongest knot you can tie.
If you cannot tie one, DO NOT DO THIS MOD. Period.
If you have children, a wife, or haul around old people at high rates of speed and this freaks you out, DO NOT tie knots. Have them sewn to hard points and use manually tightened belts, with nice clean webbing from a quality US Manufacturer.
The most important issue, is the condition of your strap and if you buy it new, who in the USA manufactured it.
I hope this helps out the single guys on a budget and clears up what was asked.
It may be a bit ghetto for some guys. It is far better than not having one at all.
This is one of those posts, where I went long, because people, without $300.00 might ignore the problem and do nothing and drive dangerously. Seat belts only save lives, if you wear them. Old rotten belts are not going to save your life. Nylon from 1977 is the same as no seat belt at all, if the polymer is dry rotted from sun exposure. This is a serious issue.
