Increasing PWM duration

Dear All,

I have such little understanding of electronics that I thought it would be better to ask for your help rather than trying to teach myself.

My enquiry will be childs play for you guys, so here goes:-

I have a motorcycle with a heated seat(< 1 amp) which is powered by 12v PWM which has 4 settings

of say 5% - 10% - 25% - 50%, controllable from the handlebars.

I would like to use the seat PWM supply to power a heated jacket (12v, 5 amp) which currently has a PWM controller outputting 50% - 75% - 100%. (BTW 100% gets far too hot)

The challenge therefore is to control a MOSFET or SSR to avoid overloading the seat supply, but to increase the PWM duration by a factor by ≈1.8.

Being fitted on a motorcycle, it would also need to be compact and durable.

All suggestions will be gratefully received.

Many thanks