I went to Lowes for the parts. Here is what was required.
3 1.5 inch 5/16" carriage bolts.
3 Rubber Well Nuts, 1/4-20. Found these in the hardware dept. in a drawer.
1 pack of stainless steel 1/4-20 Acorn Nuts. (these were the most expensive item)

First I removed the rack from the trunk then removed the right speaker. I then unbolted the mounts from the base and separated the speaker base from the cabinet. I then cleaned he mounts using a bench wire wheel. I then drilled a hole the size of the diameter of the carriage bolt.

I then mounted the pieces and used them as templates to drill holes in the plastic base.

This is where it got interesting. The carriage bolt heads stuck up too high to allow the speakers to close once mounted so I took them back to the bench grinder and ground the heads down as thin as I could get them, about the thickness of a piece of cardboard.

I then drilled out the hole in the plastic base just large enough to accomodate the square base under the head of the bolts.

(continued in Part 2.)