Just ‘finished’ (are Smart Car mods ever really done?) a stereo upgrade.
Here are the details.
Head unit (already installed by previous owner): ACIV BT90 http://www.pioneerelectronics.com/PUSA/Car/GPS-Navigation/AVIC-F90BT
Amp: KTP-445U – Alpine 4-Channel 45W RMS X 4: http://www.amazon.com/gp/product/B00866IQ2Y/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1
Speakers: Polk Audio DB6501: http://www.amazon.com/gp/product/B000P0R6LQ/ref=oh_details_o07_s00_i00?ie=UTF8&psc=1
Subwoofer: JBL GTO804 : http://www.amazon.com/gp/product/B000FHAXPA/ref=oh_details_o07_s00_i01?ie=UTF8&psc=1
Subwoofer box: Smartmadness: http://smartmadness.com/shop/index.php?route=product/product&product_id=901
So, it all went very straight forward. Got it done in about 6 hours.
As you know, I have done a few installs over the years in my different cars, and I am comfortable with car wiring and multimeters and such.
Pulled the door skins off (YouTube coming, but there are some existing ones out there), put the main speakers in. 
Drilled a round hole and then used the dremel to make the tweeters fit in the corner of each door trim.
Really glad that I did it this way, the soundstage with the tweeters mounted angled in like this is just fantastic.
Switched the crossovers for -3db to the tweeters and to my ears thats about right, would not like to run them at 0db, it would just be too bright.
Subwoofer was very straight forward. I went with a different woofer as the one Smartmadness supplied did not get very good reviews, seemed over priced and the one I chose is designed for small enclosures.
Amp. Most of the Smart Car guys have been mounting a pretty big amp under the front passenger seat…..In the end I was not happy with the mounting of said amp under the front seat. Ended up dropping the amount of power to 45w and going with the much smaller Alpine amp. Very happy with both the resulting sound and more than happy with the ease of mounting the amp.
It just sits beautifully behind the head unit. Makes for a very very clean install.
Its held down with two zip ties.
To be clear, you can not use this amp in a 2.1 configuration. You can either bridge both channels and make it a solid stereo amp, or not.
I ended up driving the Smartmadness sub with one channel and the under seat sub with the other channel. Thumps plenty hard.
All said and done, the head unit is 15 watts (at best) and sounded like crap with the factory speakers. 9/10 would not install that unit again. You can’t see the screen with the top down, it takes ~40 seconds to boot every time and is hit and miss with the touch screen.
The head unit was a lot better with it going into the Polks, but in the end, the added lift from the amp and sub(s) just make the little Smart Car rock.
It goes plenty hard, is super clear with a nice bottom end. The EQ is for the most part flat. Slight lift at the bottom (hey, I listen to Vocal Trance a lot) and mids and its beautiful. For my more mellow Oxygene (Jean Michel Jarre), its just beautiful. I go and sit in the car just to listen to music. Its the best stereo in the house!
Now I can drive with the top down and really enjoy my tunes!
Here is the full album of the install if you want to see the whole deal.
