Hello Alex,
Welcome to the forum.
I have never made a Garmin-compatible map so congratulations to you for making one so good!!!
I know a little bit about map data and I think it is the way you have defined the attributes for some of the objects in the map. For instance, the "black" background in Mobile PC might be because you have made the map "transparent". The reason I think so is that Mobile PC is capable of "stacking" maps so that a transparent map will allow the next map underneath to show through. Without another map to show through, you will see the "black" background of your monitor screen. Mapsource cannot stack maps so it will fill in a "transparent" background.
But I could also be wrong because I'm only guessing. I have no experience. We know that Mobile PC does not always display things the same way as Mapsource. It might be that the background attribute you have defined in a way that causes Mobile PC to display it differently than Mapsource.
Here are a couple of links that might help you:
Custom Maps for Garmin GPS Receivers the Elsinga family homepage » FAQ: how to create custom Garmin maps
I found these by searching for "
how to create a garmin map". There were lots more search results. I hope these are helpful.
...ken...