I feel for you Trace, I had a similar problem on my old Capri, It turned out to be the fuel pump relay not clicking off so the pump would be trying to keep the fuel rail pressurised all night. It took me months to track that down. As a stop gap I wired a motor sport type power cut off switch up next to the battery just so I knew it would start every day.
On the '5 the ECU seems to control everything like that so I doubt it’s the same problem but it might be worth getting the engine management interrogated to see if it got any fault codes logged in there.
I also agree with Jim, The best thing to do is find a good auto electrician, If you sell the car without telling the buyer about the fault you could get serious grief over it, But if you tell them, Then the price they pay will have to reflect the fact it’s got a problem.
Just don’t take the third option here of getting it fixed them selling it before anything else go wrong, It’s a thing all my friends have done once or twice and it annoys me. You spend the cash then you use the car, You don’t pay out to give someone else the benifit… Thats just mad.
OK, Here are some things to think about,
Does it still have the standard radio fitted? If not, Who fitted it? Was it just a simple plug in job or have you got a tangled mess of insulation tape behind it? Does your car have an aftermarket audio amplifier fitted anywhere? Does it turn off with the radio?
Are there any nonstandard electrical items fitted? Does all the standard electrical items work as they should (i.e. Lights, horn, heater fan, electric windows and electric aerial if fitted…)?
After a run, If you turn the engine off, Will the starter spin the engine over at a reasonable speed? If not, Are the engines and battery’s earth cables OK? How far is your drive on most trips? Less than 4 miles?
I just found something of interest on the MX5 parts website with reference to the Westco battery’s, (http://www.mx5parts.co.uk/instructions/westco.pdf). Was the battery fully charged when you got it?
I hope you find the answer before you get too disillusioned with the car. As I said, I’ve been in the “Will it start today” situation as well and it’s too easy to loose all Faith and feel that you can’t rely on it any more, Even after the problems been rectified.
Everythings fixable, You just need to know what to fix.
So come on, Chin up kiddo.