Have installed new battery after 90 K on 99 Forester. Immediately on connecting positive lead, the tail lights and amber flashers started. Then they stopped after 30 flashes. Then then lights flashed again. I put the old battery back in and the same situation occurs.
Does anyone have any suggestions?

When the battery is disconnected then reconnected the keyless entry/security system comes back on in panic/alarm mode. You have to press the unlock button on the remote to cancel it, or possibly find the switch under the dash somewhere.