Mine failed like that about 4 years ago.
Although it's a sealed unit, with judicious use of a sharp knife & other leavering implements, you can get the lens unit apart where the two halves are glued together in manufacture.
The power driver had failed on the PCB, but as they are micro sized chips I didn't think it was possible to renew the chips easily.
I bought a dozen 12 volt high brightness red LED's off eBay similar to these -
https://www.ebay.co.uk/itm/12V-Pre-Wired-Ultrabright-Waterclear-5mm-LED-Various-Colours-LEDs/261319713773?var=560239458731&hash=item3cd7de63ed%3Ag%3ATRkAAOxyAc1STZBL&LH_PrefLoc=1&_sop=15&_sacat=181880&_nkw=5mm+red+12v+leds+10&_from=R40&rt=ncBought a plastic screw connector block ( strip of 12 which you cut 2 off ) from Wilko for about £1.
You will also need a multimeter to identify the connections on the PCB & a soldering iron.
Identify the 12v positive ( with multimeter ) on the PCB where the wire from the bikes wiring plugs in, the one that becomes live when you operate the brake.
Soldered a red wire to that.
Identify the earth ( Negative ) on the PCB, solder a black wire to that.
Screw the other end of both wires into the connector block.
Then screw all you individule LED wires into the other side of the connector block, obviously red to red & black to black.
Glue the LED's into the lens unit, just a tiny bit of superglue or soft plastics glue.
Test it on the bike, then use some soft plastic clear glue to fix the two halves of the light unit back together.
Mine has not given any problem since.