Hi Amy and welcome. This is a great site and there is tons and TONS of info on it - the search engine is massively useful.
I used Michael's instructions for fitting 3 point belts, which he's sending to you. Since I've just done it, the job is still clear in my small brain. I've been asked by another member to write down a few comments about the job, which isn't hard, so here goes:
I have a US '67 230SL. My comments relate to my car and my experiences; your car may be different. My car has no back seat and has a soft top - this is relevant as cars that are different to mine will need some welding to be done to affix a the bar mentioned in the instructions - if your car is like mine, you don't need to buy the bar and you won't need to weld anything.
1. Get Michael's MB club instructions Ref MBNA 91/15 dated May 89. Get the seatbelt kit and parts. I got mine from SLS in Hamburg for about $200. You will also need 'threaded plates' which are also available from MB and are part of the seatbelt kit. (NB - the MB instructions say you need 4 of them; if you do this the way I did, you only need two).
2. Cut the plates as shown in Fig 1b - the one where you just cut off the top hole (ignore the more complicated plate it says you need to cut if your car is like mine - those are the plates you DON'T need).
3. Remove the carpet in the area you need to work, and the wooden shelves. Remove the trim covering the B pillar (6 screws, I think). On my car, the fittings for the 3 point belt were already there - these are shown in the figure 3 of the instructions. I used the hole directly by the number '3' in figure 3 of the instructions to attach my shoulder loop to. It is designed for that purpose; I've discussed it with SLS in Hamburg and they say its also fine. The 17mm bolts that come with the kit fit, as that's what that hole is for.
4. You will need to drill the holes in the bottom of the B pillar at the floor as set out in figure 5 (para 10 refers). Then you slide the plate you've cut into the area and secure it with a small screw - these don't come with the fitting kit - just use anything to hold the modified plate in place. The 17mm bolt is the one that takes the strain. The dimensions given in figure 10 are spot on - use them. If your car is like mine, ignore fig 5 and paras 10.1, 11, and 12, which are for a different car. (No welding required for this job).
5. Make the cuts as at figure 6/para 13. Use an angle grinder if possible as the metal is of good quality. I was saddened to see that I'd have to cut through the area that was marked with the check mark of the car assembler, but there was no alternative; it's a shame to destroy these uniqe signs of originality, but there we are. You will need to trim the vinyl and re-glue. Ensure you leave yourself enough to be able to secure the ninyl properly when you stick it back on. I cout away too much in a hurry.
6. Dry fit everything and check it's OK. The angle of the seatbelt at the shoulder will be a bit odd - if you want to make it better you will have to cut into the top of the B-pillar as shown in the instructions, but I have decided that I can like with it for now.
7. Before you fit everything, coat all cut/drilled surfaces with anti-rust paint. WHen fitting the belt rollers, put some locktite on the bolt threads to stop them coming undone; also do this at the shoulders.
8. Fit the belt locks (the thing the belt plugs into over your lap) to the correct position either side of the transmission tunnel. The holes should already be there - use a thin nail to find the hole if you have no belts there already. (Make sure you buy the shortest lock - SLS sell a 12mm long one - that's the size you need; anything longer and you'll break your rubs if you're in a smash).
9. Fit it all together. Put the B pillar covers back on and the wooden shelving - you'll see that the belt rollers are concealed neatly under the shelves - I put a thin piece of rubber tubing where the belt would rub against the vinyl on the B pillar trim to save wear on the belt and the vinyl.
That's about it. I avoided the welding and cutting into the car by using the existing holes. I admit that the sholder thingy is not perfect given the angle it sits at, but when the belt is on, it's as good as anything. I suggest you check out how it will sit and see what you want to do.
I'll post some photos soon.
Good luck.
James
