If you have a SD card based Satnav system that runs Windows CE then here's an idea that worked for me....
Pretty much wasted an entire weekend doing it but VERY happy with the end result. ( OziExplorerCE



Have done this to a couple of different brands now and so far no tears. ( Bill, Joey - we have the same $150 laser units )
Copy your SD card - Don't trash original card.
Install OziExplorer CE on the new card -download is free and is completely functional apart from a "demo Message" and a click every 10 or so minutes )
Delete original SatNav files to free up disk space( BUT note directrory and executable name )
Rename OziExplorer Directory to SatNave directory name
Rename OziExplorer executable to SatNave executable name. ( need to select ARM or MIPS version - delete unneeded )
Bingo - swap cards and when you select satnav instead of your original program OziCE will fire up.....
Obviously you now need OziExplorer compatible maps - but they are out there......

Things to watch....
Some SatNavs don't support bigger SD cards 1 GB upper limit seems common. Bigger is better but .......
Use Microsoft ActiveSync ( freeware download ) to copy files - a standard card reader gave me all sorts of problems.
Also managed to bypass fixed menu to get usual WindowsCE - Start Menu - Programs..... this allows for Windows style manual program selection BUT if the battery runs down or the unit is reset this facility is lost until its "refixed" and this requires a PC to do.
This is a technical limitation of Windows CE, having no harddrive CE is stored in flash memory and transfered to RAM when rebooted.
So in the bush it MIGHT cause a problem if you have a major system issue, its unlikely but .....
Given the 1GB card limit on mine, seperate cards seemed the best and most technically robust solution., this way the CE flash issue can't cause problems.
Also transfered all service manuals onto a seperate smaller card with a free PDF reader ( Foxit ).
Its small but you can read what you need with a bit of work. Only for emergencies but usefull.
So - there you go, a VERY satisfactory modification, but its a "at you own risk" modification. Shouldn't cause a problem as your playing with a copied replacable SD card but its your risk.