DECLARE
	v_car_no NUMBER;
BEGIN
	SELECT count(*)
	INTO v_car_no
	FROM you_table_name
	WHERE ROUND(TRUNC(car_expdate)) - TRUNC(sysdate) <= 7
	IF v_car_no > 0 THEN
		Msg_Alert('Display your Alert here');
	END IF;
END;

SQL> CREATE TABLE test
  2  (car_no NUMBER,
  3   car_regdate DATE,
  4   exp_date DATE);

Table created.
SQL> INSERT INTO test
  2  VALUES(1, '12-JAN-2006', '25-JUN-2006');

1 row created.
SQL> INSERT INTO test
  2  VALUES(2,'12-JAN-2006', '25-SEP-2006');

1 row created.
SQL> INSERT INTO test
  2  VALUES(3, '12-JAN-2006', '20-JUN-2006');

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM test;

    CAR_NO CAR_REGDATE        EXP_DATE
---------- ------------------ ------------------
         1 12/JAN/06          25/JUN/06
         2 12/JAN/06          25/SEP/06
         3 12/JAN/06          20/JUN/06

Code in When-New-Form-Instance Trigger:

DECLARE
	v_car_no NUMBER;
	CURSOR c1 IS
	SELECT car_no
	FROM test
	WHERE ROUND(TRUNC(exp_date)) - TRUNC(sysdate) <= 7;
	c2 c1%ROWTYPE;
BEGIN
	SELECT count(*)
	INTO v_car_no
	FROM test
	WHERE ROUND(TRUNC(exp_date)) - TRUNC(sysdate) <= 7;
	IF v_car_no > 0 THEN
		OPEN c1;
		LOOP FETCH c1 INTO c2;
			MESSAGE(c2.car_no||' is going to expire');
			EXIT WHEN c1%NOTFOUND;
		END LOOP;
		CLOSE c1;
	END IF;
END;