Stereo & Electronics - Convertible Monsoon doesn't like Infinity 62.7i?

I installed left side replacing a blown stock speaker and
when I start listening it's OK, but the left volume keeps
going lower and lower. Next day (like a cooldown has
happened) it starts off OK again and fades shortly.

The voice coil resistances measure out to roughly 2
ohms on the Monsoon and the other Infinity I have
out. So it doesn't seem like there ought to be a
difference but the right side (unblown Monsoon) ends
up working loud while the Infinity has gone nearly
inaudible after a while (the tweeter still twets but the
mid is not pumping anymore).

I installed left side replacing a blown stock speaker and
when I start listening it's OK, but the left volume keeps
going lower and lower. Next day (like a cooldown has
happened) it starts off OK again and fades shortly.

The voice coil resistances measure out to roughly 2
ohms on the Monsoon and the other Infinity I have
out. So it doesn't seem like there ought to be a
difference but the right side (unblown Monsoon) ends
up working loud while the Infinity has gone nearly
inaudible after a while (the tweeter still twets but the
mid is not pumping anymore).
Which wires did you connect the new speaker to? The factory system is separate components mounted coaxially rather than a normal coaxial speaker. So you should have found four wires - two for the mids and two for the tweeters. If you hooked up the tweeter wires you wouldn't get much signal - try the other pair.

That's wierd that it's basically fading away. Could be a bad speaker or a channel in the amp it'self. Hard to test and tell.

Try testing the speaker itself. When it's warm/hot and the sound has faded take another measurement. I have TS intermittent coil's on circuit cards that displayed a similar effect and it was basically a completed ciruit when the component was cold, but as it warmed up, the coil would start to seperate enought that resistance would increase and start attenuating the ciruit path.

Trick to finding out that this was happening was I took a can of air and turned it upside-down and sprayed the coil. Ciruit would act fine after that. But I don't think you'd be able to do something like that with a speaker.

You could also just swap sides with the mids and see if it still happens. If the problems moves, it's a bad speaker.