Buying a Supabrake 3 is the easiest way to get brake-light modulators going, just plug it in between the front and rear harness. Comes with OEM connectors, literally a 10 second install job. A bit pricey but good stuff.
I wanted more brake light too but didn't want to add too much extra stuff back there so I just used the indicators and added some LED clusters. Made a big difference in visibility rearward, easier to see it's a bike with three lights than just one.