@wvy01

First - The best practice for this kind of maps is to place them in a memory card. If your unit becomes inoperable and you have to send it to Garmin it will not be advisable to leave obvious evidence of "free" maps inside the main unit.
It will of course work fine also to use the maps in main memory, but be aware of possible problems. If something is wrong with a certain map on a memory card just remove the card and reboot the unit. If the problem is in main memory you might end up with a annoying hard reset instead.

Second - Since all CX maps for nüvi 37xx here uses MapID=800 & PID=1 the it will be easier to create just one unlock code for all maps here. Just use the file [Unit drive or memory card drive]:\Garmin\gmapsupp.unl" to store the unlock code.
In this case you can have Madrid, London, Paris etc. without the need to create UNL-files for each of them. The "gmapsupp.unl" file can store multiples of 25-char codes but you only need one code for all 800/1 maps. If you want to store other codes for other maps also inside "gmapsupp.unl" they should be continously added without delimiters or CR/LF.
All this is already described in [Only registered and activated users can see links. ]

EDIT - update: The usage of "gmapsupp.unl" to handle all CX MMR-data unlocks for FID=800 might not work any longer. In that case you just need to copy your CX_some_city.unl file to CX_some_other_city.unl.

Third - If you're using a memory card and don't know the CID (card ID). Press the battery icon in upper right corner (~8 seconds) to enable diagnostics mode.
Press "Next" twice and look at the "Data Card Test Page". Read the "Serial" data to be used as CID. Use CID instead of UnitID with JM KG.


To test the cityXplorer Madrid you either have to have a satellite fix in that area or simulate a current position in the coverage area.