Ok, here's me clutching at straws:-
1. Take out the stereo you installed or at least remove it's fuses to see if this eliminates the problem, I favour taking it out very carefully and check to see you haven't shorted, crushed or disconnected something.
2. This is a real long shot, the mileage is stuck at 52999, did this happen immediately after you put the stereo in? If so my money is on something in No1 above, if it happened later then there is a remote possibility that the speedo has jammed and is throwing error codes and blowing a fuse but as i say it's a really long shot. If it as me I would have retraced all of my steps regarding the stereo installation it's almost too coincidental, hard to know what else to suggest so good luck with it.
In the unlikely event that one of these is the solution please donate the £40 to Hope House Hospice or the Forum