Does anybody have any idea why it would do this? Also - the negative wire that on the AEM install diagram says should go to the ECU, can it just be ground or is it necessary, and does it go right below the positive, which is at 2p?

 

go to flyin miata an download the LINK install pdf. The 2p is supposed to be moved to 2 R I believe.

here Flyin' Miata : Instructions : ECUs

On my AEM I don't believe the previous owner grounded it to the harness. He grounded to the common signal ground on the cylinder head. The install manual mentions where to run this ground. Then there is another ground that went to the chassis.