Why change to LEDs? Two reasons, as they do not have a filament as incandescent bulbs do, they do not suffer failure due to vibration, and they draw very little power as they do not produce much heat.
The standard Ducati ST has a very good rear light with a great reflector design as will be seen in the photos. The downside is that it only has one bulb that operates the rear and brake light (5 watt and 21 watt), two separate lights make the brake light function more obvious to following vehicles.
I will apologise for the pictures, but taking photos of lights is difficult as the camera wants to compensate for differing light levels. That means that when one picture shows the rear light and the next picture the brake light, it is difficult to see the difference as the camera has dulled the much brighter brake light.
A selection of bulbs that I will try. 1. is the standard incandescent bulb, it has two fagile filaments, one for rear and one for brake light. 2. is an LED cluster bulb with all the LEDs facing outwards, the disadvantage of this one is that it does not use the reflector. 3. is an array of LEDs facing backwards with three rings of LEDs facing the reflector. 4. is a panel of nine 5050 red LEds and 5. is a panel of 2836 white LEDs.
Bulbs 1, 2, and 3 all fit into the standard rear light with no modification. All the LEDs are for sale on ebay, normally from China, for less than £5 each.
Before we look at comparisons, this is what is in the back of my bike, this will be the lower light in all of the pictures. The red mark shows the two red LED panels one above the other that operate as the rear light. The green marks show the two white LED panels that are the brake light. The white shows through the rear lens as red, but is a slightly different red to distinguish itself as a brake light. These panels can be arranged differently and I could have fitted four reds at the left and right with the two brake panels in the middle if I had wanted. In this light the reflector has been cut away and the LED panels fitted to a bit of perspex that is glued to the remains of the reflector. Three wires protrude from the rear which plug into the standard bike wires.
And this is bulb 2. as brake light.
This is bulb number 3, as brake light, I messed up the photo as rear light, but it matters not as you can see what a great use it makes of the reflector.
If I did not have the time or the knowledge to play with these things than I would spend £2 on pair of these EBAY LINK and have one as a spare. As I do have the time to play and I like to see more lights come on when braking, I went for the whole panel. If I had my panniers fitted all the time I would also build additional slimline LED lights into the back of those and have a waterproof plug on the rack.