Best way to clear grit and sand out of 7 pin plug??

I would suggest first blow out as much as possible with air.
Next submerge in Simple green without diluting it. Leave in the Simple green for several hours or if you have the time a full day.
Next blow out the simple green with air then use hot water to remove the rest of the Simple Green. Then blow out with air until completely dry.

As stated get a cover for the plug when not is use. I found one at my local NAPA Auto Supply. 7 way rubber boot