Most importantly, these brines are denser than ice
Like seawater, the brines drain downwards, clean fresh ice is found at the surface.
If you want to find liquids on Mars, you gotta drill, and the SHARAD and MARSIS radar results tell us you gotta drill DEEP.
of course, nature has a way of confounding us, whenever we think something is simple, but let's start simple and work up, rather than start with something complicated, like perchlorate brines at surface.