relays are essentially an electronic switch, when a voltage is applied to the coil it will close the switch when it is removed it will open (normally open relay), without going into too much detail the type of relay youre talking about would be set up to cut its own power supply to make the light flash, as such if it is working you should be able to hear it clicking in and out, if it inst (with you’re multi meter in dc mode because its being fed by a battery) check there is power getting to it, if there is then you would expect continuity (complete circuit) through one side when the coil is energised and not the other and vice versa when it is de energised - but if you’re not familiar with electrics i would just listen for the clicking, if its not clicking its either not getting power for some reason or its stuck/burned