Turn Signals
It can be done with switches, that "old school" switch is large and clumsy.
As far as I know you need to use a relay with a switch for each direction (or a return to center momentary) trigger the relay to dump the brake signal, and feed the flasher through on the other side of the switch. In other words, a DPDT center off momentary plus 2 relays
With all the home built hotrods (roadsters, etc) there is very likely a cool "turn key" product, but I don't know "whut"