I had the same question regarding the correct polarity but here's what I did:

Before desoldering the stock speaker leads from the pod I took a note of which wire is connected to which terminal in the pod. Since the polarity on the stock speakers are marked, I now know which pod terminal is positive and which is negative.

I then connected a 1.5v battery between the terminals of the stock speaker making sure to match the speaker terminal marked positive (+) with the battery positive (+). I then took a note of how the cone moved (either in/suck or out/push).

I then connected the 1.5v battery to the Rainbow speaker and if the cone moved the same way as the stock speaker, I marked the terminal connected to the battery positive (+) as the speaker positive (+) terminal.

Following this process ensured that all the speakers are installed in-phase.

I have found that there is no need to remove the vertical sunshade when installing the speakers in the rear panels. Its tight but entirely possible with a shorty screwdriver.