After the kicker speaker upgrade, it was obvious that a separate amplifier was needed. A regular recommendation is the Soundstream Picasso Nano 640D that I believe has recently been discontinued. I found a used unit with all the attachment cables on Ebay and ordered. 5 channels, small footprint and plenty of power.
The next obstacle was my 10+ year old head unit. It had 2 pairs of pre-amp RCA outs; one for the front speakers and another set that could be set to rear or the sub. There may be other solutions, but I opted to replace the unit with another forum recommendation: the Kenwood KMM-X705.
The Kenwood install was easier expected. Crutchfield supplied all of the required parts. I had it done in about an hour. No real change in sound quality running from the built in amp vs the old Pioneer.
The amp install was much more exciting. There are plenty of good examples here (1,2) for mounting the amp in the unused space under the steering column. The forum research and pre-planning made this install about as simple as it could be. I spread it out over a few evenings and had it ready to go for the weekend.
I’ve only done some basic tuning and I am blown away. The sound is clean, the tweeters up front are fantastic, the $25 Goldwood sub sounds great and I can hear everything cruising with the top down.