All you need is an electronic flasher relay with all cars from the mid 90's use (plenty on eBay) and wire it as so.
Make an earth wire to a good earthing point and put it on terminal 31
With a test lamp (or multimeter) switch the ignition on and find the permanent live to your current flasher relay, connect to pin 49, then the other wire connect it to the remaining pin 49a.
As for the tell tale if a bulb is blown, that's just tiger99 scare mongering yet again. You don't have any warning systems or lights for the brake lights, so how would you know (without getting out to check) that your brake light/s have just failed? The only reason why the remaining indicator stays on with one less bulb working ,is there isn't enough current through the flasher unit to heat up the strip and brake the contact.