Actually it's quite easy to replace the seat belt holder red PRESS button caps. Even the local yard didn't consider it doable. Pick up a used seat belt holder of similar vintage (front, back, they all use the same red button). Should be very cheap. Just get a nice bright red one that isn't sun faded. These red PRESS button caps have tabs (less than 1/2" long) extending down from each end with a square hole in the tab that simply clips over a small stud on the button underneath to hold them in place. Draw a (mental) line through the word PRESS and the tab holes will be out at the edges, straight down about 1/4". To remove a button, use a thin, small slot screwdriver. Pry under the high side (not the ends), above and well to the left of the "P" then above and well to the right of the "S". Pry up one end a bit then the other, then go back and pry up the first side until it pops off its tab then pry the other end off its tab and the button will be in your hand. They're quite sturdy as long as you pry them straight up. To install, just push straight down. I gave up trying to keep the lights working.
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Dave -940's, prev 740/240/140/120 Never overestimate what little I know.