You don't
have to replace both Bulbs,
but it's often best to, as if the first one has failed due to its age,
then its likely the other one will go the same way soon !
The Headlamp bulbs are standard Tungsten Halogen H4 twin filament bulbs,
and any make can be used, though you do tend to get what you pay for in terms of quality.
If the bulbs are really old, then even when replaced with the same brandyou might
might notice a slight difference / colour variation.
Philips make some nice bulbs, Extreme Vision / White Vision etc.
If you want to go down the LED road, they need to be H4 compatible,
and Legal for use in your Country / State.
Check their Heat Output, and that any Heat Sink will preferably dissipate heat
away from the plastic headlamp unit which
can melt if it gets too hot !
( A good reason for not leaving the Bike running at standstill for prolonged periods ! )
Heated Grips are still available from Triumph for either side :
Hermys Triumph eStore for the Triumph TrophyI have NO idea why they list two variants of Heated Grip for Left and Right.

What is NOT available from Triumph is the "Connector Pack" ( A963002 ).
This is the white nylon housing that the two wires from the Heated Grip are pushed and locked into,
AFTER they have been fitted to the Handlebars, and which are then pushed into the corresponding
socket on the Trophy wiring harness.
You could either carefully remove the Connector Pack from your existing faulty Grip and re-use it,
or cut them off the new Grip AND the Trophy harness and make you own new connection,
or if you want to keep the Bike in as original condition as possible, I have sourced the "Connector Packs"
from an external supplier, though the Shipping price from UK to US will bump the price up !
Details here :
HEATED GRIPS "Unobtainable" Parts For Sale !Cheers
