Hello I had the same problem on my mad maxi I ended up rewiring the whole ped including a new switch.
I use 6v 18/18 wat bulb at the front and a 12v 4v bulb in the rear.
This has been tried and tested now for over a year as i use the ped to got to work in all wether. I do how ever always have a spare bulbs just in case, you never know!
The high volt read bulb will act as a resistor and put more current though the front bulb so to stop the front blowing you will need a higher watt bulb up front.
Also when switching from high to low, there will be a over surge to the rear bulb
The rear bulb is always on when the lights are on but the front will momentarily
be without power when switching, this will in turn give the all the lighting power to the rear bulb! that's about 17watts !
I found that even with the 12v rear bulb it would blow. It was only when i did the rewire and new switch it stopped blowing the 12v bulb.
It still blows the 6v ones even now.
I hope all this helps :)