I would suggest following the linked guide with a few changes / notes below : https://www.fiat500owners.com/threads/how-to-2012-fiat-500-double-din-radio-install.7412/

1. The double din dash adapter seems a bit iffy from the Amazon reviews, YMMV: https://www.amazon.com/Scosche-FT4276B-Double-Select-Vehicles/dp/B00RL9L9F8/ref=sr_1_1?keywords=FT4276B&qid=1573525758&sr=8-1
2. Antenna Adapter on Amazon for ~10 bucks: https://www.amazon.com/gp/product/B07DWLTP6R/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
3. For the headunit I'm assuming you'd want bluetooth and Android Auto / Apple Carplay? I would highly suggest the Kenwood DMX706S : https://www.amazon.com/gp/product/B07SMBMQ5Z/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
4. To use the head unit after install without actually splicing your parking brake and other cables, get the Kenwood bypass cable: https://www.amazon.com/gp/product/B01FY8P9QC/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1
5. To tie all this together, you'll need the wiring harness. I suggest idatamaestro:https://www.amazon.com/gp/product/B00E4W1644/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1 https://www.amazon.com/gp/product/B012F8RC1Y/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1 You will make an account and flash the firmware to the appropriate car make/year and head unit model.

I've only installed a basic 1 DIN stereo on my old 2013 500. I suggested the above head unit and wiring harness because I've installed that on my 2017 500 and had great results. Best of luck.