I remember seeing a solar recharger (or power adapter) made by Sony that was for operating Walkman's.

Since Walkman's are probably low (electrical) draw items, what I would do is read the specifications on the solar device as to output. See how many milliamps (MA) it puts out.

Make sure it it puts out the amount you need for the GPS. The Manual for the GPS should tell you this somewhere in the technical data section. (front or back) Garmin has good gear and probably makes a solar charger.

Also the solar charger may put out a leser amount but by the same token it may put out some of the necessary juice so that the installed batteries last longer. I would ask the manufacturer if this is possble. It may be that when you hook up an auxiliary power source the main pack is disconnected.

Even if the output of the solar device is less than the amount required to charge it, it may have enough juice to operate it.

Make absolutely certain you get the polarities correct when connecting the two devices.

Another way to skin the cat is to buy rechargeable battery packs that have a lot bigger batteries with the same voltage as the ones you put in the gps. Lots of model radio controlled aircraft/boat shops/sites and rechargeable battery resellers will gladly make you up a blister pack with batteries that will last as long as you could ever want them and you would just get an adapter power cord to hook it up to the GPS. The GPS manufacturer may even sell them.

Most regular AA cells would have (I am guessing) about approximately somewhat maybe 500 to 1000 MA of juice. If you use the battery up in 10 hours you are using (for instance @1000 MA battery) 1000 divided by 10 = 100MA per hour. so if you buy a battery pack with all 4000 MA batteries you get 4 times the life.

Also not all batteries are created equal. NiCad's are cheaper and come in a wide range of output. You can get fast charging ones that have low output (Ma) or slow charging ones that have lots of juice. The fast charge ones are OK with getting overheated. Some missile batteries can be charged in minutes.

NiMH batteries are slow charging and supposedly give out more juice as much as 50%, although I've heard some people say this is not true. Maybe someone else can advise.

In my hang gliding days we used to make up battery packs which we would remote into our pod harness for long days going cross country.

Make all connections very high quality! Better still get a professional (such as a marine) electronics guy to make up the connections as salt and or water is very corrosive and you don't need a battery /GPS failure. Poor connections can be a pain as they cause gremlins.