Can you show us a couple of pages where these things appear? Use the camera icon on the Web page to embed a screen capture, like this:
I suspect though, that the simple answer is that your cars and trucks tags appear on each page, and you've set the TOC to find their paragraph style. Is each category in an isolated section, or are they spread around and mixed together? If they're in sections, you can add a "category tag" on the first page of each section, just a frame (and set it to non-printing and make the type big and red so you see it using some unique paragraph style not used elsewhere) with the exact text you want to appear in the TOC for each category, then include that style in the TOC.
OK, but are the products sorted? Are all Trucks together and all Cars together (even if spanning multiple pages)? As long as they aren't mixed in together you can manually add the category tags after the merge.
If they're all mixed together, I think you need to use an Index, rather than a TOC.