You can get a combined Power/Data cable for the Garmin 12 for a few extra pounds over the price of the data cable I find it very useful and good value for money
NB. Cables for Garmin 12 and Garmin 12XL are not directly interchangeable as they have different power requirements.
A data cable for a 12XL can be made to fit the 12 with the help of a small drill. I would not recommend trying to do the same thing with the power or power and data cables.
NB 2 You can of course make a data connection without having to buy the (expensive) Garmin data lead by connecting directly to the pins at the back of the unit. It's a bit fiddly (try using turntable cartridge leads) but fine for occasional use
If you're in to DIY you can make up your own cable for the 12 by using components bought from <A target="_blank" HREF=http://pfranc.com/projects/g45contr/g45_idx.htm>http://pfranc.com/projects/g45contr/g45_idx.htm</A>. Costs a few pounds rather than the £30 or so for the pucker unit. I bought on so I could have the GPS on deck and link it to my PC at the chart table. The Garmin cable is only a few feet long and I needed a run of about 10 meters. Works fine.
I recently bought one from Russell Simpson at Brighton Marina. I also noticed that alot of the chandlers at Southampton Boat show had them, so I guess they are widely available by mail order. Be warned. They are not cheap!