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I've often thought about this but I have never found a mountable female receptacle that is small enough to be discreet. If you could find a small receptacle and then get a male plug-in to go with it you could mount it flush with the outside of the wheelwell. Maybe Radio Shack???
On my 1990 Vert; I installed a s/steel grid under the nose to stop stuff getting inbetween the rads. This allowed me to drill a hole in it and put a grommet in it. I then ran wiring from the battery thru the hole with the plugin thru the grid. I have my Battery Tender(acually the AutoMeter one) mounted on my garage wall with a extension to plug into the one under the nose. To keep the plugin under the nose from dangling; I used one of those small plastic wire clip holders connected to the s/steel grid and just hooks onto it. I did make a cap to plug into the plugin on the car. It makes for a easy plug in when I pull into the garage after a cruise.
On my C5 I hard wired a Battery Tender Plus' cord to my battery. Then used their optional extended cord to route the plug coming out where the Driver's side hood and fender meet. I just pull in the garage, plug it in and I'm done. I'm sure the same routing can be done on the C4, no holes need to be cut, and you can route it on either side of the car depending on where the charger is mounted in your garage.
I just bought a battery tender and installed the "pig tail" to the battery using GM Post Adapters from NAPA. Th adapters are brass and cost about $5. They replace the stock battery cable bolts. I then ran the other end of the "pig tail" down and out of one of the gills on my '89. Pull in the garage, grab the plug out from the gill and plug it into the tender. Couldn't be easier.
I believe Mr. Mojo ran his wire out through the gill panel. You can pull the wire out and plug it in and when you're done you tuck it back in the through the gill panel.