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I have read on a few forums where people have used this antenna base and trimmed the top element so that it was tuned to the CB with a
SWR meter. Has anyone here got this to work? We have been working on this but ran into some issues which may have to do with grounding.
I would be interested in hearing about some lessons learned or advice with this. Thanks

It will actually need to be longer. Not shorter. The AB-15 as used in our trucks is 6m or 50Mhz. CB is 11m or 27Mhz.
A 1/4 wave for CB is 102" long.
In general, you will need to be resonant in order to appear as a 50ohm impedence to your radio and keep your VSWR below 2.5:1.
You can run just about any length as an end fed antenna IF you have an impedence match between the antenna and radio. Then you tune the antenna to the R and VSWR. It won't hurt the radio this way, but only testing or analyzing will determine how well or if it will radiate.
Using the stock approximate 56" antenna is close to 1/8 wave. But far enough off you will not get a good VSWR. (Because it will become very capacitive and the R will be around 5-7.)

There and different antenna section that thread into the Ab-15th. generally the MS-116, Ms-117- and MS- 118 sections are the most common ones out there. You can mix and match to get the length you need. If you search high and low you may find other sections like the Ms-121 and 122, but they are rare and hard to find. But, by putting the different sections together you should get to the 102 inch length that Just Me talked about. The AB-15 with the three section 116-118 is meant to cover 118-136 MHz where as CB is at 27 as stated above. So, yes when you were told to cut to length was correct, but you need the right combo of sections to be able to cut to length.

Although it requires some knowledge and external gear to use initially, something like this will match the antenna to the radio. Set it up for the channel you will run and don't touch it again.
http://www.palcoelectronics.com/p1003949.aspx

Years ago, I had instructions for making your own helical wound CB antenna. It allows you to get a good match without having to go to the full 102" length. I made one by wrapping wire around a fiberglass wand. I've long since lost the instructions, and this was more than 40 years ago (back when I had a Ham license and was up on this sort of thing). But I do wonder if making one of these to fit the original mount would work.

Yeah, just winding wire around a form is harder to do than it would seem on the surface. Following instructions from someone who did the hard work is a great option.
Base loading, center loading and tip loading all have their own pluses and minuses.
If I was to try and use the AB base for CB, I think the easiest way would be to use a Wilson 1000 short version and modify the AB to screw in the stick.
I can come up with several ways a clever fellow with minimal tools could do successfully.
A Firestick could also screw into an adapted AB. That would be easy to paint OD to look correct.
Tune the length for a good SWR and good to go.