my detachable plates hit my factory seat on my '95 wide glide too, but not my corbin. I ride regularly with both seats and it's never caused a problem. Either pull the side plates out, slide them over the seat, and push them back in, or install the side plates separately then install the backrest.

BTW, thanks for your service over there, stay safe!

 

My Mustang pillion is actually 13" wide. Try the following...

It's a little awkward, but I've done it many times like that also.