My apologies to all. Apparently I confused everyone in my problem description. Removing the panel liner and replacing the pushbutton wasn't an issue once the glass hatch and tailgate door were opened.

The problem I had (yes, it's fixed now) was the tailgate and glass hatch were both closed. The pushbutton was broken and would not allow unlatching the glass hatch so the tailgate door could be opened, which was needed to remove the inside tailgate panel liner, which was needed to replace the button, etc, etc.

The find...

On Van's advice, I first went to myisuzuparts.com, but couldn't locate the pushbutton lock assembly in their drawings. Next I called the parts guy at the local Isuzu dealer and guess what, he couldn't locate the assembly in his system either.
He called Isuzu, left a message with them and said he would get back to me. That was 3 days ago and I haven't heard from him since.

Being somewhat impatient, I decided to call around the junkyards, er used parts suppliers <img src="/forums/images/graemlins/smile.gif" alt="" />, and eventually found a used one. A bit of a drive and I don't have a key for the lock, but at least it's repaired and I'm not waiting on Isuzu. The lack of a key wasn't an issue anyway as the Rodeo has power door locks and I rarely use a key.

The fix...

On the advice of the local Isuzu mechanic, I removed the inside liner and unbolted the top hinges for the glass hatch. That was absolutely worthless as it gave me no more room to access the latching mechanism.

After deciding not to cut a hole in the tailgate door liner, (definitely a last gasp effort!!) I decided to push the button completely into the tailgate door cavity. (next to last gasp effort!! )
I was then able to fish around with a long skinny screwdriver and bring the latching rod up into view so I could grab it with a pair of long skinny needle nose pliers. .....and then pulled it to unlatch the glass hatch. <img src="/forums/images/graemlins/kewl.gif" alt="" /> Once opened, the rest of the job was extremely simple.

Thanks to all for the advice. I hope my experience helps someone else out there!